Air Pressure: Balloon in a Flask

Put a small amount of water in the bottom of a flask.

Place the flask over a burner or on a hot plate until the water begins to boil.

Turn off the heat and stretch a balloon over the flask opening.

Allow the flask to cool.  You can speed this up by placing the whole flask in ice water. 

The balloon is pushed into the flask.

Why:

When the water is heated, the molecules speed up and many of the escape the flask.  The balloon, placed over the opening, prevents the air molecules from re-entering the flask as it cools.  In addition, there are lots of air molecules on the outside of the flask and balloon, and they no longer meet much resistance, so they push the balloon into the flask.

If you're at home, and don't have a flask or a burner, try this:

Use your tea kettle (or other pan) to heat some water until it begins to boil.  Quickly pour the water into a bottle (you could use plastic since you're nto heating the bottle) - use a funnel - and cap the bottle with a balloon.  Then, proceed as above, cooling the bottle.