This experiment really challenges kids to think about things they can't see - namely air. Both my boys were impressed.
Even I was impressed.
What You Need
A drinking glass
Paper or paper towel
Tub or bowl of water
What to Do
Crumple the paper and push it tightly into the glass. Turn the glass upside down and plunge the glass into the tub of water. Keep it there for a little while.
Pull it out.
Inspect the paper. Why isn't it wet?
Water couldn't get into the glass because it was already full ... with the paper and air. Because the air is lighter than water, it can't escape.
This experiment comes from the book Science Play! by Jill Frankel Hauser.