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Mystery Upside Down Glass of Water - Air Blasters! - & Enviroscope
What you will need for this experiment:

12-ounce plastic cup
Water
Sink, Bathtub, or large container to catch any water spill
5"x8" index card or plastic lid (cottage cheese lid)

What to do:

1. Fill the cup almost to the top with water.
2. Place the lid over the mouth of the cup.
3. Gently hold the cup in one hand and hold the lid in place with the other hand.
4. Over the sink, turn the glass upside down with one hand while still holding the lid in place with the other hand.
5. Once the cup is completely upside down, slowly remove the hand that is holding the lid in place.

Can you figure out why the lid stayed stuck underneath the cup? Try this experiment with larger size plastic cups or containers, larger lids, and see if there is any difference between the sizes used.
How it works What mysterious, invisible force holds the lid in place? It’s air pressure, Air pressure is the force exerted on you and everything around you by the weight of tiny particles of air (mostly oxygen and nitrogen molecules). Although air molecules are invisible, they still have weight and take up space. For more information and activites on air pressure go to:
http://kids.earth.nasa.gov/archive/air_pressure/

Last Updated on August 6, 2002
Questions?? Contact ewong@arb.ca.gov
A department of the California Environmental Protection Agency