In today's task, Why is the Water in Glass?, we use another magic trick of sorts—a piece of paper holding water in a glass turned upside down, seemingly on it's own—as the phenomenon to motivate student sensemaking. Students engage in the science and engineering practice of modeling and use the thinking tool of cause and effect (crosscutting concept) to make sense and/or use science ideas about air-as-stuff and forces.

Be prepared to get a little wet with this task!

Materials

• transparent glass or plastic cup
• transparent plastic cup with small pinhole in the bottom
• plastic cup with small pinhole in the side of the cup (near the bottom)
• stiff paper squares slightly larger than the mouth of the cups you are using (file folder, cereal box cardboard, playing cards, etc.)
• water
• large bowl or bucket (to catch spills)
• OPTIONAL: additional liquids (oil, rubbing alcohol, milk)

Watch the Mysterious Water Suspension video above to learn how set up the upside-down glass of water. You may have to practice. Make sure you use a new paper square each time.

Now try using the cup with the pinhole. You should find that the water spills out of the cup when you turn it upside down (it make take a few seconds).

Say to students, "I saw this really puzzling phenomenon I want to share with you!" Show students how to set up the upside-down glass of water, then turn the glass upside down. Slowly remove your hand from the paper square.

Ask, "What are your ideas about how to explain this phenomenon?" Give students time to independently record ideas in their science notebook or on blank paper. Encourage them to use words, pictures, and symbols.

Next, ask students to turn and talk with a partner to share their ideas. If students don't have experience with student-to-student discussion, you might give them these partner conversational supports.

As you walk around the room, listen for students to share ideas about air being all around the glass, gravity pulling on the water and paper square, and/or balanced forces. Watch for students who are gesturing something pushing up on the paper square.

Give students two or three minutes to change or add to their ideas in their science notebook.

Bring the students back together. Draw the upside-down glass of water on poster paper (or on a whiteboard). Ask students, "What do you think is holding the water in the glass?" (The paper square). "How is the paper square holding the water in the glass?"

Ask the students you heard talking about gravity and forces to share what force(s) they think are acting on the paper square. Students will likely cite gravity or the weight of the water pushing down. Draw an arrow from the paper square pointing downward and label it weight of water.

Ask the class, "Based on the model, what is the motion of the paper square: upward, downward, or not moving?" (Downward!) "How did the card move when I turned the drinking glass upside down?" (It didn't move.) "How could we show on our model that the paper square is not moving?" (Draw an arrow up.) "What size should the arrow be?" (The same size as the down arrow) If students don't agree whether there should be an arrow, what direction it should point, or the size of the arrow, ask if it's acceptable to draw a question mark next to the arrow.

Say, "We seem to agree that the forces acting on the paper square are the same size, but in opposite directions." If students have experience with forces acting on objects at rest, you might remind them of those experiences (a book on a table, students seated in a chair). "What forces are acting on this book?" (gravity pulling the book down and the table pushing the book up) "What forces are acting on you?" (gravity pulling me down, the chair pushing me up).

Ask, "What is pushing up on the card?" (air, the water makes the paper square stick to the glass). Ask the students you heard talking about air to share their ideas with the class. You might ask where you should draw air on the model. Draw air particles on the model. Students might not agree that air is pushing up on the card even if they agree that something is pushing up. You might add another question mark to the model next to the up arrow.

You might ask, "How could we figure out if water is making the paper square stick to the drinking glass?" Students will likely suggest trying to turn the glass upside down with a different liquid.

Repeat the procedure for the upside-down glass of water, but this time use oil or another liquid like oil. Ask students, "Does our model predict what will happen when we use other liquids?" (yes) "Do we agree a force is pushing up on the paper square? May I erase this question mark?"

Tell students you want to show them the upside-down glass of water one more time, this time using a cup with a small hole in the bottom. Hold the cup, using your finger to cover the hole. Ask a student to fill the cup with water, then place a paper square over the top of the cup.

Before you turn the cup upside down, ask students to predict what will happen. Give students time to independently think and record their ideas. Then ask students to share their ideas in small groups. You might use the partner conversational supports in the small group as well. Consider asking one group member to be the speaker and the rest of the group to take turns as the responder.

Bring the class back together. Ask for three students to share their predictions. Students can share their own prediction or a group member's prediction. Ask students to explain their prediction using observations from the upside-down glass of water or the class model. You might use the talk move, "Would you say a little bit more about that?" to encourage students to elaborate on their ideas.

Turn the glass of water upside down, remove your hand from the card, and ask students to make observations. Did their observations match their predictions? Give students time to independently think and record changes or add to their ideas. Ask students to turn and talk with a partner to share why they changed or added to their ideas. Give students one minute to share, then switch roles.

Draw a second model next to the first one, this time representing the cup with the hole. Ask students, "What forces are acting on the paper square?" (gravity/weight of water, air) "How should I draw the arrow representing the air pushing the card up?" (the same as before/air didn't change). "How should I draw the arrow representing the weight on the card?" (bigger/thicker/longer because the card moved down) "Why is the weight greater in the cup with the hole?" Accept all ideas.

If students aren't in agreement that the weight increased because both water and air pushed down on the paper square, ask them if anything else changed besides letting air into the cup. (You might have to do the upside-down glass of water demonstration in a plastic cup if you didn't do it initially.)

Ask students, "What do you think would happen if I made a hole in the side of the cup?" Let students turn and talk with a partner before asking them to share ideas with the class. Repeat the upside-down water in the glass demonstration one more time, this time using the cup with a hole in the side. (Note: Make sure when you turn the glass upside down that the hole is above the water line.)

Say to students, "Why do you think the water didn't stay in the cup?" (Air got in again.) "Are we in agreement that air is all around and pushes on objects in all directions?"

Air pressure can do more than hold water in an upside-down drinking glass! Share the Imagine It: Put an Egg in a Jar Using Air Pressure video with students. For younger students, you might explain that the scientist is using different methods to change the force air is applying to the bottle from the inside (allow students to figure out the relative forces acting on the egg). Older students might understand how changing the temperature of the air inside the bottle changes the force air is applying to the bottle from the insides.

Consider showing 0:50 to 1:12 without sound and asking students to explain the phenomenon using observations from the upside-down glass of water and class models.