Here's a science trick that will teach you all about air pressure.
* Plastic bottle with cap
STEP 1: Using the nail, make a hole in the side of the plastic bottle.
STEP 2: Use your finger to cover the hole as you completely fill the plastic bottle with water.
STEP 3: Screw the cap on the top of the plastic bottle and then remove your finger.
Since you screwed the cap on the plastic bottle you stopped the air on the outside of the bottle from pushing down on the water inside the bottle. This is the reason why the water did not rush out of the bottle when you removed your finger. Unscrew the cap and the air from outside of the bottle will force the water out of the hole.
Jason Lindsey is a Science Outreach Educator for the Bootheel Youth Museum. Check out his website "Hooked On Science" at hookedonscience.org for webcasts and experiments that might get you hooked on science. Send him your science questions at firstname.lastname@example.org. More science experiments can be found at www.semissourian.com.