Balloon Trick

What You Need:

• 2 plastic two-liter bottles
• 2 round balloons
• Permanent marker
• Ruler
• Thumbtack

What You Do:

1. Before you start the activity with your child, poke the thumbtack multiple times to make a small hole an inch or two above the bottom of one of the soda bottles. 
2. Tell your child you're going to have a contest to see who can blow up a balloon first.
3. In order to measure when the balloon is fully blown up, both you and your child will blow it up from inside a plastic soda bottle.
4. Have your child help you place one deflated balloon in each plastic bottle. Don't drop the balloon all the way into the bottle! Keep the mouth of the balloon poking out.
5. Fold the edges of the balloon's mouth over the mouth of the bottle. If you do it right, the balloon should stay on its own.
6. Assign one bottle to your child, and one to yourself. To complete the trick, make sure you get the bottle with the hole poked near the bottom!
7. Have your child use a permanent marker and a ruler to mark a point on each bottle about about seven inches from the bottom. This will be the mark both of you will "race" to— and the first balloon to reach the mark wins.
8. On the count of three, both you and your child should start blowing up your balloons.
9. What happens? While your balloon should inflate without a problem, no matter what your child does, her balloon just won't inflate!
10. Let her guess why this happens before showing her the hole you poked at the bottom of your bottle. Why would that make a difference?
11. Explain that this trick works because of air pressure. Since there is already air inside of the soda bottle, her repeated efforts to add more air to the balloon will never work. The air needs an escape route—something like a tiny poked hole—to allow the balloon to inflate.
12. Now that your child is in on the trick, have her try it out on her friends!