When Lincoln and I learned about weather a few weeks ago, I checked out a DVD at the library on storms. From that DVD, Linc picked up the idea of air pressure. He's always talking about air pressure, and filling his toys in the bath tub up with water and (after putting the lid on) tipping them over so no water spills out and saying, "Look! Air pressure!" So Aaron finally decided to show him something that really was air pressure. To repeat what he did, take an empty pop can and fill it with a 1/4 of an inch of water, and then put it on the stove until it boils (steam will come out the top). Once it starts boiling, pick up the can with tongs, and turn it over so the hole is face down, into a bowl of ice water. The can will crush. (And make a very satisfying crunching noise.)
When the water in the can boils, steam replaces the air in the can. When you put the mouth of the can in the bowl of ice water, the steam inside the can condenses back into water, leaving a vacuum inside the can. Then the pressure of the air outside the can crushes the can. Air pressure! (as Lincoln would say.) They had a really fun time with this experiment.
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