Free mini-lab · about 5 minutes · ages 5 and up
The upside-down water cup
Turn a full cup of water upside down, and the air around you holds the water in.
You need
- A clear plastic cup
- Water
- An index card
- A sink or tray
Predict first
What will happen when you let go of the card? Have everyone guess before you try.
Try it
- 1Fill a clear plastic cup to the very top with water, over a sink or a tray.
- 2Lay an index card (or a piece of thin cardboard) flat on top, so it covers the whole rim.
- 3Press the card down with your palm and turn the cup upside down in one smooth move.
- 4Slowly take your hand away. The card stays, and the water stays in the cup.
- 5Tilt the cup a little. Then try it with the cup only half full, and compare.
What's going on
Air is pushing on everything, all the time, from every direction. When you flip the cup, the water sags a tiny bit and the little bit of air inside spreads out, so it pushes down less. Now the air outside pushes up on the card harder than the air and water inside push down, and a thin film of water seals the card to the rim. With more air in the cup it can still work, but it's trickier, so try both and compare. That same push, air pressure, is what held a car to the ceiling of a curved ramp.
Grown-ups, stay close: practice over a sink, and use a plastic cup, never glass.
Want the full hour? This lab is part of Rocket Roadster Science Lab. See the lesson