LEAKY BOTTLE
Gravity is pushing downward on the water whether the lid is on the bottle or not. Air pressure can't do anything until it somehow gets into the bottle. In other words, when the cap is on, air pressure can't get into the bottle to push on the surface of the water. It does, however, push on all sides of the bottle but since the atmospheric pressure is greater than the force of gravity, most of the water stays in the bottle.
When you take the cap off of the bottle, the atmospheric pressure and gravity push down on the water at the same time. The water has no where else to go but out of the hole that you made in the bottom.
Play this prank on somebody with a good sense of humour.
What you'll need:
Empty plastic soda bottle (l-litre size works well), cap (the bottle lid), large nail
What you do:
Use the nail to poke a hole on the side of the bottle but near the bottom. You should probably ask an adult to help you make the hole so you don't hurt yourself.
Cover the hole while you fill the bottle up with water all the way to the top and then screw on the cap. Slowly take your finger away from the hole. Notice how no water leaks out (do not squeeze the bottle at this point). Place the bottle either in the fridge or offer it to your unsuspecting victim. When they unscrew the cap, you will witness science in motion and they will get an unexpected drenching.