use this on ur friends to scare them, they will back away and cover their face once they see it pointing at them. its a lame instrutable (1 pic, pretty obvious how to do) but hey, it might help some people (sorry for only 1 pic...didnt feel like taking pictures of a bottle =)
Step 1: Get Bottle
just find any bottle. some bottles will work, and some wont. it should be about a 17 fl. oz. bottle. (bottle of water etc.)
Step 2: Loosen & Tighten
first take the cap off and let out a tiny bit of air, then put the cap back on just far enought so that its right on the edge of letting air out, and not letting air out. (basically tighten the cap just far enough so that it doesnt leak air.)
Step 3: Twist
twist the bottle so that lots of pressure builds up. twist as far as possible and hold there.
Step 4: Open Cap / Finish!
now while holding it twisted, point it at sombody and loosen the cap very quickly. the cap will pop off and hit them. woo! yes i know this is a pretty lame instructable but whatever. (you can also do the same thing but tighten the cap a little more, and jump on it.)