We’re on a science kick at the moment.  Here’s another easy experiment.
You need a small glass (we used a small empty Gerber container). a piece of cardboard to cover the glass, and water.
I highly recommend doing this over the sink, just in case.
1. Fill your glass completely with water.
2. Place the cardboard over the glass.
3. Hold the cardboard and turn the glass upside down.
Ask your little one to predict what will happen when you remove your hand.
Remove your hand from under the cardboard and be totally amazed that the cardboard stays there.  The air pressure pushes up on the cardboard and keeps it there!