clear plastic bottle with cap

water

Sharpie marker

something sharp to poke a hole with

Procedure:   Poke a small hole in the bottle about halfway up from the bottom.  Draw a face on the bottle using a Sharpie.  Be sure the mouth is puckered and is centered over the hole you just made; this will be where the bottle "spits."  With your finger covering the hole, fill the bottle to the top with water, then tightly screw on the cap.  When you release your finger, no water should come out.  To impress your friends and family, say, "Magic bottle, there you sit!  On the count of 3, I'll make you spit!"  Then count to 3, and on 3, unscrew the cap.  Water will begin to come out through the hole!  Tighten the cap and the water will stop.  You can repeat over and over until the water level is below the hole.

The Science:  When the cap is on the bottle, the air pressure inside and outside the bottle is the same, so there is no force to push the water out of the hole.  When the cap is loosened, however, air rushes into the bottle, which increases the pressure inside the bottle and forces the water out of the hole.  When the cap is tightened again, the air pressure equalizes again, and the water stops flowing.