Magic Ballon

What you need

a clear plastic bottle (like a 1.5 or 2 litre fizzy drink bottle)
a pen
a balloon (blow it up a few times beforehand)

What to do

Make a small hole in the bottom of the bottle with the pen.
Push the balloon inside the bottle through the neck of the bottle and stretch the neck of the balloon over the mouth of the bottle.
Blow up the balloon.
Notice air is coming out of the hole in the bottle.
Cover the hole with your finger and stop blowing.

What is happening

The balloon stays inflated! As the balloon expanded, it pushed air out of the bottle. That made the pressure inside the bottle lower than the pressure inside the balloon, so it wasn't strong enough to squeeze the air out.