Water Bottle Trick: Air Pressure & Gravity

It's the ultimate prank using the simplest of props... a plastic soda bottle filled with water. Even though the words, "DO NOT OPEN" are printed on the bottle, people just can't resist the temptation.

What you need:
2 plastic soda bottles (1-liter size works well),
cap
permanent ink pen
thumb tack (push pin)

The Leak-Proof Bottle
Use the tack to poke a few holes on the side of the bottle close to the bottom. (Get an adult to help with this part!)

Put the bottle in a sink or bowl filled with water. Fill the bottle with water and keep the water running into the bottle as you screw on the cap.
Lift the bottle out of the sink being careful not to squeeze it. What happens?
Unscrew the cap and watch what happens.

The "Do Not Open" Bottle Trick
Start with a new plastic soda bottle (don't use the one from the previous experiment). Write “DO NOT OPEN!” with the permanent ink pen on the bottle.
Fill up the bottle as before.

Set the bottle on the kitchen counter (word-side out) where someone can see it as they pass by. Stay close enough to watch what happens. Eventually, someone is bound to ask about the bottle. Let them unscrew the cap and you'll witness science in action!The science:The science of it is simple with gravity and air pressure acting on the water only once the bottle is opened. Gravity is pushing downward. However, when the lid is on the gravity cannot push on the water surface only the lid. It does push against the outside of the bottle on all sides.Since the outside atmospheric pressure is greater than the force of gravity, most of the water stays in the bottle. Once the lid is removed, gravity and atmospheric pressure will act at the same time on the water surface. The water will leak out. Scientific proof was never so much fun.