experiments on cohesion

Science in School

Fantastic feats: experimenting with water teach article.

Author(s): David Featonby

How can air hold the water in an upturned glass? Why does water stay in a bottle with a hole in its base? Find out with these entertaining experiments.

From their earliest years, children enjoy playing with water, and so do many older students. In this set of experiments, we look at the forces that are significant when dealing with water, demonstrating some basic science principles – and some surprising results. All the experiments are safe to do at home as well as at school, and require just simple household objects as the equipment, plus plenty of water.

Experiment 1: the upside-down glass

Many people have tried this experiment in some version, but can you work out what’s really going on?

• Straight-sided water glass
• Piece of thin card (large enough to cover the open end of the glass)
• Pour water into the glass until it is nearly full.
• Place the piece of card on top of the beaker.
• Turn the beaker upside down with one hand, holding the card in place with the other hand.
• Remove the hand holding the card (figure 1).
• Note what happens. Does the card fall off and the water fall out? Can you explain why not?

Surprisingly, when the glass is inverted, the card and the water remain in place. Why is this?

Let’s consider the forces on the card. These are:

• gravity, from the weight of the card itself (acting downwards)
• gravity, from the weight of the water pushing on the card (acting downwards)
• air pressure, which pushes on the outer surface of the glass and card, acting at 90 0 to the surface of the card (so producing an upward force on the card where this has just water above it).

So in this experiment, the force of air pressure pushes upwards on the card at the open end of the glass, opposing the force of gravity and keeping the water in the glass.

Extension: estimating the upward and downward forces

How do we know that the upward force of air pressure is enough to oppose the downward force gravity, to hold the water in the glass? We can estimate these forces quite easily.

The weight of the card is much less than that of the water, so to simplify we can ignore the weight of the card itself. This means that the downward gravitational force on the card is the weight of the water column

= h x A x ρ x g

where h is the height of the water column, A is the cross-sectional area of the glass, ρ  is the density of water (1 000 kg/m 3 ) and g is the acceleration due to gravity (approximately 10 m/s 2 ).

So if h is 10 cm (0.1 m) and A is approximately 25 cm 2 (0.0025 m 2 ), the downward force is approximately

0.1 x 0.0025  x 1000 x 10  = 2.5 N

For the upward force, this is the atmospheric pressure, P , multiplied by the area over which it acts, A.

Atmospheric pressure is approximately 100 000 Pa (pascals, or N/m 2 ).

So the upward force on card = P  x A

= 100 000 x 0.0025  = 250 N

So for a 10 cm water column, the upward force due to the atmosphere on the card far exceeds the downward force of gravity on the card due to the water.

We also need to recognize that the air above the water plays a significant role.  If this remained at atmospheric pressure, the weight of the water would be sufficient to remove the card, however as soon as the water exerts a downward pressure on the card, it reduces the pressure of this trapped air, which is sufficient to enable the upward atmospheric pressure on the card to support the water. A 1/100 change in volume of this air is sufficient to balance the water, i.e., the pressure reduces by 1/100 th  which is equivalent to the pressure of the water.

Further investigation

You can also think about the questions below, and perhaps carry out further experiments to answer some of them:

• Does this experiment work if the glass is completely full of water?
• How does the ratio of air to water change the experiment outcome?
• Would this experiment still work, no matter how tall the glass is?
• What other shaped containers (e.g., bottles) can be used?

Experiment 2: water’s invisible ‘skin’

In this experiment, we discover how cohesive forces within water act like an invisible ‘skin’ that can keep the liquid in an upturned cup – sometimes.

• Piece of thin woven nylon cloth (large enough to cover the open end of the cup)
• Elastic band
• Thin card (large enough to cover the open end of the cup)
• Cover the open end of the cup with the nylon cloth (figure 2, left).
• Pull the cloth tight, and secure it with the elastic band (or glue it to the cup around the rim).
• Pour water into the cup through the cloth, nearly filling it.
• Place the card over the nylon and the open end of the cup.
• Turn the cup upside down.
• Note what happens: the water should stay in the cup, as in experiment 1.
• Now carefully remove the card. Does the water flow out through the nylon cloth? If not, why? Water was poured in through the cloth, so why doesn’t it pour out again?
• To pour the water out turn the cup upright again quickly, then tip up the cup slowly while pressing a finger on the nylon (figure 2, right).

The reason why the water does not flow out through the very small holes in the nylon is because there are forces of cohesion between the molecules in the water. These forces make the surface of the water act like a ‘skin’ between the tiny holes in the nylon cloth. This effect is known as surface tension, and it is the same principle that keeps you dry under a woven nylon umbrella: there are tiny holes in the cloth, but the rain won’t get through due to the cohesive forces of surface tension between water molecules.

Further investigations

There are plenty more experiments you can do with surface tension and molecular cohesion. Perhaps look up ‘surface tension experiments’ on the internet and see what other activities you can find?

Here are two further simple experiments you can try.

Paperclip boat

Take a dish of clean water and a paper clip. Hold the paperclip in a strip of paper towel and lower it into the water. Then allow the paper to sink or carefully sink it with a toothpick. The paperclip will appear to float but is in fact being held by the surface tension of the water.

What else can you ‘float’ – for example, a ring pull from a drinks can? What happens to the paper clip if a drop of washing-up liquid is added to the water? How can you explain what you see?

This effect can also be used to make a ‘ soap boat ’.

Joined-up water jets

Take a clean empty drinks can, plastic cup, or bottle and make three small holes close together, near the base. Fill the can with water, and when three jets come out, use your fingers to try and join the jets together. You will be able to do this, because of the cohesive forces between water molecules.

Another fun experiment to illustrate cohesion is pouring water down a string .

Experiment 3: bottled water

These activities use a simple bottle of water to reveal some surprising effects due to surface tension and gravity. It’s a good idea to do them outdoors because water spillage is likely (see figure 3).

• Plastic bottle (250 ml) with screw cap
• Large needle or nail

Use the needle or nail to make one (or more) very small holes near the base of the plastic bottle by heating it over a flame (safely held) until it is hot enough to melt the plastic. In schools, this should be done by the teacher in advance of the experiment, for safely.

• For fun, you can add a label to the bottle saying ‘Do Not Open’ – and see if people ignore this.
• With the cap off, quickly fill the bottle with water, holding your finger over the hole, and then replace the cap.
• Hold the bottle still (or hand it to someone else) with the cap closed. What happens to the water?
• If the warning is ignored and the bottle cap is opened, what happens?

Once a container is sealed, water will only flow out of a small hole if that water can be replaced by air or more water. A bottle with one small hole can therefore hold water if the cap is sealed. Once the cap is unscrewed the water will flow out, due to the weight of the water. The hole needs to be small enough for the surface forces to hold the water.

Another interesting experiment to try with a full bottle of water with a hole near the base is: what happens when you throw the bottle up and catch it?

If you fill the bottle with water and hold it with the cap unscrewed for a few seconds (figure 4), the water will flow out of the hole.

Now throw the bottle up in the air (figure 5), and watch it carefully as it falls. Observe each part of its journey – on the way up, at the top of its flight, and on the way down.

When the water is in free fall (i.e. on the way down), water will cease to flow out of the bottle. This is because the water within the bottle becomes weightless relative to the bottle itself, as both the bottle and its contents are in free fall. Thus, in this situation the weight of the water does not force it out of the bottle.

This effect can also be demonstrated with a water-filled hollow tube (around 50 cm, with a diameter that can easily be covered by a finger). The bottom of the tube is covered with one hand while it is launched into the air, with this hand exerting the launching force and the other just supporting the tube. However, this can be a little tricky to demonstrate because it is essential that the hand covering the tube end is the last to let go when throwing the tube up and the first to contact the tube again on catching it, to avoid accelerating/decelerating the tube without the water column.

You can also try to answer these final questions:

• What happens to the water when the bottle is travelling upwards during the throw? Can you explain this?
• If you try to catch the bottle, what happens to the water? Can you explain this?
• What else can you throw in the air so that there is a change in what happens when it is in free fall, compared with when it is stationary? Hint: Think of toys or devices that work with gravity, e.g. where particles or moving parts or liquid fall through a gap.
• A more detailed version of the thrown water bottle experiment: Tsakmaki P, Koumaras P (2017)  When things don’t fall: the counter-intuitive physics of balanced forces . Science in School 39 :36–39
• Try a similar experiment to the paper cup and nylon cloth activity:  https://www.stevespanglerscience.com/lab/experiments/water-screen/
• Watch this video with more activities to try with your students using water:  https://www.youtube.com/watch?v=CCxbI1qRsWY&ab_channel=DrewtheScienceDude
• Learn how to make a soap boat:  https://www.youtube.com/watch?v=OU76wwmg9Hs
• Watch a video on the running water experiment:  https://www.youtube.com/watch?v=8nOU7jbRPPo&ab_channel=DrBoydTheChemist
• Read other Teach articles from the Fantastic Feats series:
• Featonby D (2017)  Fantastic feats.   Science in School 39 :45–47
• Featonby D (2018)  Further fantastic feats: falling and bouncing .  Science in School   43 :37–54
• Featonby D (2019)  Fantastic feats: magic with money .  Science in School   47 :46–50

David Featonby taught physics throughout his career in a large UK comprehensive school, and now shares his ideas across Europe through the organisation Science on Stage, of which he is a board member, helping to organize its activities. He has presented workshops in various European countries and written articles for both Science in School and Physics Education , including a regular series called What Happens Next? in the latter. David has a particular interest in making physics relevant to all ages through experiments that use everyday equipment.

The simple experiences shown in the article, easy to reproduce and using materials that are easy to find, allow students to approach the topics of surface tension and pressure in liquids. The author, also, by highlighting the “magic side” of some experiments, makes them more interesting and also suitable for the general public. The article offers the possibility to make interdisciplinary links to biology topics, such as pulmonary respiration (and how nature provides the alveoli with surfactants to decrease surface tension), capillarity in plants, or how the surface tension allows some insects to walk on the surface of the water

Maria Teresa Gallo, Math and science teacher, Italy

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Water buoyancy, cohesion & adhesion.

September 9, 2014 by Marjorie Frank

Among other things, water is a super resource for science activities. You might be surprised at the amount of learning that can come from one tub of the wet stuff.

Buoyancy is the upward force on an object that is produced by a surrounding gas or liquid, such as water. You’ll find a sentence similar to this in many science textbooks. But what does it really mean—water produces an upward force—huh? Most students probably will read a sentence like this and move on without pausing to think about it. Here’s an activity to bring the sentence, and the concept of buoyancy, to life:

To prepare, challenge pairs of students to engage in a friendly game of arm wrestling. Then discuss the experience. Did students feel the force of their opponent pushing against their own efforts? Now, challenge students to submerge a beach ball in a tub or sink of water and describe how that experience compares to arm wrestling. Speaking from personal experience, it can be a real challenge. For sure, students should feel the force of the water pushing up against the ball just as they felt the force of their arm-wrestling opponent. Yes, water does exert an upward force.

Sinking and Floating

Water buoyancy is connected to sinking and floating, a phenomenon your students probably are familiar with. When the downward force of gravity is greater than the upward force of a liquid, an object sinks; otherwise it floats. Many middle school science books explore this concept in terms of density: Objects that are less dense than water float; other objects sink.

What happens when the water is in solution with salt? Sugar? Food coloring? Baking soda? How is density affected? To find out, fill a few glasses half full with water. Place an egg in each glass and ask students to describe what happens. (The egg sinks.) Enlist volunteers to stir in tablespoons of salt to one glass, and tablespoons of other materials to each of the other glasses. Urge them to keep track of how much of each material they add and to observe the egg as they go along. What happens to the egg in each solution? What can students infer about the density of each solution, based on their observations? Now, ask for a prediction: What if students used an eyedropper to gently drop a water-and-food-coloring solution onto the salt water—would the colored water sink or float? (It floats.)

Cohesion, Adhesion

Whether an object sinks or floats is connected to its density and to the tendency of water molecules to stick together. To explore, fill a bowl with water. Then challenge students to test different ways to put paper clips on the water’s surface so they float. Guide students to apply the concept of buoyancy to help explain why laying a paper clip flat on the surface enables it to float while standing it on end or laying it on an angle causes the paper clip to sink.

Point out that a second phenomenon is also at work: cohesion. Water molecules cohere or stick together, forming a kind of skin called surface tension. Surface tension enables some insects to actually walk across a pond without sinking. To investigate further, have students repeat the paperclip activity with alcohol instead of water and observe the results.

Cohesion is a useful property, but it has its limits—luckily. If there weren’t a stronger force than cohesion, how would water travel from the roots of a tree upward, against the force of gravity, to the tree’s tallest branches? If water only “stuck” to itself, it would stay in the tree’s roots indefinitely.

Lucky for us, adhesion is stronger than cohesion. Adhesion causes two different materials to stick together. To see the force of adhesion at work, have students roll and flatten a sheet of paper towel. Suggest they place one end in a glass of water and the other in an empty glass an inch or so away. Ask students what they think will happen. During the course of a day, encourage students to observe. The water will travel up through the paper towel into the empty glass. That’s the very process that takes place when water travels up the roots to the stems and leaves of a plant. It’s called capillary action, and it’s based on water molecules adhering to the molecules that comprise the paper towel (or plant stem).

When it comes to cool activities with water, this is only the tip of the iceberg (pun intended). You could probably build an entire science curriculum around water! If that idea has appeal, check out this site for lots more possibilities: http://pbskids.org/zoom/activities/sci/.

It’s true that in some parts of the country water is in seriously short supply, but each glassful can go a long, long way in science. It may well be worth it.

Marjorie Frank A writer and poet by nature, an educator and linguist by training, Marjorie Frank has authored a generation of instructional materials for children of all ages, including songs, poems, stories, games, information articles and teaching guides. Marjorie has two grown children, Adam and Ben. She currently lives with an artist (whose work you can see in the Kids Discover issue on Plants) and a dog that looks like a pig.

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• February 2017
• Foundations

19th Century Experiments Explained How Trees Lift Water

A maple branch and shattered equipment led to the cohesion-tension theory, the counterintuitive claim that water’s movement against gravity involves no action by trees..

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FREE K-12 standards-aligned STEM

curriculum for educators everywhere!

Find more at TeachEngineering.org .

• TeachEngineering
• Balancing Liquid on a Coin: How Intermolecular Forces Work

Hands-on Activity Balancing Liquid on a Coin: How Intermolecular Forces Work

Grade Level: 10 (9-11)

(or two 45-minute classes)

Expendable Cost/Group: US $1.00

Group Size: 2

Activity Dependency: None

Subject Areas: Chemistry, Physical Science, Problem Solving, Science and Technology

NGSS Performance Expectations:

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Engineering connection, learning objectives, materials list, worksheets and attachments, more curriculum like this, pre-req knowledge, introduction/motivation, vocabulary/definitions, activity extensions, user comments & tips.

Wastewater engineers work to ensure that all water systems at a water treatment plant are working correctly. Their responsibilities revolve around producing clean, safe drinking water for the community. Their duties include the design and operation of the machines, systems, and equipment that receive, clean, and distribute water. Knowing the basic properties of water is an important first step in developing water treatment methods.

After this activity, students should be able to:

• Explain why water is important to life.
• Explain how the properties of water and isopropyl alcohol differ due to their molecular structures.
• Consider how the properties of water make it unique and why it has a higher evaporation point than alcohol.
• Have a cursory knowledge of water’s structure to consider its properties for future engineering.

Educational Standards Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards. All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN) , a project of D2L (www.achievementstandards.org). In the ASN, standards are hierarchically structured: first by source; e.g. , by state; within source by type; e.g. , science or mathematics; within type by subtype, then by grade, etc .

Ngss: next generation science standards - science, international technology and engineering educators association - technology.

View aligned curriculum

Do you agree with this alignment? Thanks for your feedback!

Each group needs:

• science notebook for each student
• Intermolecular Forces Lab Worksheet for each student
• dropper bottle with water
• dropper bottle with isopropyl alcohol
• paper towels
• safety gloves for each student
• stopwatch, smartphone timer or watch timer

The teacher needs:

• projector and whiteboard/blackboard/chart paper for recording student ideas

Students should know how to read molecular formulas and draw atom structures. They should have learned about atomic and molecular structures as well as chemical bonding. They should understand polarity and dipole moments.

In many ways, water is a miracle liquid. It is essential for all living things on earth and it is often referred to as a universal solvent because many substances dissolve in it. Water displays unusual properties due to the ways in which individual water molecules interact with each other.

As a refresher, let’s review the properties of water. Who can name a property of water? (Answers include polarity, cohesion, adhesion, surface tension, high specific heat, and evaporative cooling.)

Today you will complete two experiments that compare the properties of water to the properties of isopropanol (a type of rubbing alcohol). Before conducting each experiment, you will make a prediction about what you think will happen. You will then read about and do the experiment. Afterwards, you will write an explanation of what was happening at the atomic level that let us observe these properties.

In many ways, water is a miracle liquid. It is essential for all living things on earth as it is one of the more abundant molecules in living cells. Approximately 60% – 70% of the human body is made up of water. Without it, life simply would not exist.

Because water is essential to life, it is important to know the basic properties of water. As it turns out, water displays some unusual properties due to the way individual water molecules interact with each other.  

A single water molecule is composed of one oxygen atom and two hydrogen atoms. Together these atoms form polar covalent bonds. (A covalent bond is a chemical bond when electrons are shared between two atoms.) A polar covalent bond forms when atoms with different electronegativities share electrons in a covalent bond. In water, the result is that each hydrogen atom shares electrons with the oxygen atom; however, the shared electrons spend more time associated with the oxygen atom than they do with hydrogen atoms. The result is that there is a slight positive charge on each hydrogen atom and a slight negative charge on the oxygen atom.

Since the hydrogen and oxygen atoms in a water molecule carry opposite (partial) charges, nearby water molecules will be attracted to each other. The attraction between the δ+ hydrogen and the δ- oxygen in adjacent molecules is called hydrogen bonding. Hydrogen bonding is an intermolecular force, which are forces between molecules in a substance. Hydrogen bonding is one of the strongest intermolecular forces. This means that neighboring water molecules are strongly attracted to one another. This is called cohesion. The property of cohesion describes the ability of water molecules to be attracted to other water molecules.

Surface Tension

Water has surface tension. Because the water molecules at the surface of liquid water have fewer neighbors, they will have an even stronger attraction to the few water molecules that are nearby. This enhanced (or greater) attraction is called surface tension. It makes the surface of the liquid water slightly more difficult to break through than the interior of the water.  Cohesion gives rise to surface tension, the capacity of a substance to withstand rupture when placed under tension or stress.

Water is often referred to as a universal solvent because many substances dissolve in it. Because of water’s polarity and structure, ionic compounds and polar molecules can readily dissolve in it. Water is, therefore, what is referred to as a solvent—a substance capable of dissolving another substance. The charged particles in water molecules will form hydrogen bonds with a surrounding layer of water molecules. When a substance readily forms hydrogen bonds with water, it can dissolve in water. Since water can dissolve so many it is important in its many roles in living systems.

Evaporation

The hydrogen bonds in water allow it to absorb and release heat energy more slowly than many other substances. Water can absorb a great deal of energy before its temperature rises.  Since the hydrogen and oxygen atoms in the molecule carry opposite partial charges, nearby water molecules are attracted to each other. The attraction between the δ+ hydrogen and the δ- oxygen in adjacent molecules is a special type of intermolecular force called hydrogen bonding that causes water molecules to “stick” together in liquid form. This force must be overcome for liquid water to become a gas. It takes a lot of energy to overcome the force of hydrogen bonding.

When enough energy has been added to water it boils and turns from liquid to gas. This happens at a temperature of 100˚C. In comparison, isopropanol turns from liquid to gas at 82.5˚C. Since the vaporization of isopropanol occurs at a lower temperature than water, this means it takes less energy to turn isopropanol into a gas, and therefore it will evaporate faster than water. Isopropanol has weaker intermolecular forces holding its molecules together, so it takes less energy (a lower temperature) to separate the molecules to enter the gas phase.

Before the Activity

• Arrange students into groups of two or let them work individually if enough materials are available. 
• Develop a system for having students get rid of lab waste and return materials to the instructor.

With Students

Part 1: Introduction (10 minutes)

• Students can work independently or with a partner through the Introduction section.
• Have students answer questions 1 and 2 of the Intermolecular Forces Lab Worksheet .
• While they work on the Introduction section, select a student or group to present their structural formulas of water and isopropyl alcohol to the class. This may be done using a document camera, or any other way you can present student work to the class. 
• Answer any questions or help correct misconceptions before having students work on Experiment #1.

Experiment #1: Penny (30 minutes)

• Hand out materials to students.
• Students work though Experiment #1 
• Place one penny on a paper towel.
• Drop water onto the surface of the penny, slowly, one drop at a time.
• Count how many drops it takes until the water spills off the penny.
• Record the number of drops to the right.
• Repeat steps A through D for isopropanol.
• Compare the number of drops of isopropanol to the number of drops of water.
• (Optional) If time, have students repeat the experiment for consistency.
• [Optional] Pause students after collecting data about number of drops of water on the penny and discuss everyone’s findings. This can be an opportunity to calculate averages and analyze the group data. 

• Have students read and mark up the “Reading” section about intermolecular forces and then develop an explanation about what happened during the experiment.
• [Optional] Have students share out their explanations and clear up misconceptions about surface tension before moving on.  Another option here is to have students define the tier three academic vocabulary (bolded) in their notes: surface tension, hydrogen bonding and intermolecular forces.

Experiment #2: Evaporation Rates (30 minutes)

• Students work though Experiment #2 

• Dip one end of a Q-tip in water. Shake off excess water.
• Dip one end of a different Q-tip in isopropanol. Shake off excess isopropanol.
• At the same time, draw (streak) the tips of each Q-tip across a paper towel in two parallel lines.
• Time how long it takes for each streak to evaporate.
• (This may take a few minutes.) Record your observations.
• [Optional] Pause students after collecting data about time taken for each liquid to evaporate and discuss findings. This can be an opportunity to calculate averages and analyze the group’s data.
• Students read and mark up the “Reading” section about hydrogen bonding and then develop an explanation about what happened during the experiment.
• [Optional] Have students share out their explanations and clear up misconceptions about hydrogen bonding.  Another option here is to have students define in their notes: surface tension, hydrogen bonding and intermolecular forces.

cohesion: Describes the ability of molecules (such as water) to be attracted to other molecules of the same makeup.

evaporation: Process by which water changes from a liquid to a gas or vapor.

hydrogen bonding: Attraction between the δ+ hydrogen and the δ- oxygen in adjacent molecules.

intermolecular force: Force between molecules in a substance.

polarity: Separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.

solvent: The liquid in which a solute is dissolved to form a solution, or the ability to dissolve other substances.

surface tension: Attractive force exerted on the surface molecules of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area.

Pre-Activity Assessment

During the Introduction activity while the students are drawing their structural models, in the teacher should walk around and see who is doing their models accurately and who needs more support. By going over the models BEFORE completing the activity, students who are struggling to understand are given a chance to revise their thinking.

Activity Embedded (Formative) Assessment

During the introductory assignment where students learn discuss the importance of water, there is an opportunity for formative assessment by walking around and listening to partnerships share out ideas about what will happen in each of the experimental trials.

Post-Activity (Summative) Assessment

During the activity, students evaluate their predictions as accurate or inaccurate and then explain what they observed after reading the section. This helps to see students’ abilities to explain the observed phenomena. 

Safety Issues

There are no safety issues, but students should wear gloves as a best practice for maintaining a safe lab space as well as follow other lab safety protocols.

This lab is a great introductory lab to the properties of water and can be used as any pre-teaching lesson before students work with the mechanics of water or other polar substances.

Students learn about the basics of molecules and how they interact with each other. They learn about the idea of polar and non-polar molecules and how they act with other fluids and surfaces. Students acquire a conceptual understanding of surfactant molecules and how they work on a molecular level. ...

Contributors

Supporting program, acknowledgements.

This material is based upon work supported by the National Science Foundation under grant no. EEC 1407165— Boston University Photonics Center Research Experience for Teachers. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: October 14, 2024