Gravity bongs can be made out of an empty 2-liter and a pitcher. Here's what you do, cut the bottom off the 2 liter bottle, fill the pitcher with water, put the 2 liter in the pitcher, take a piece of foil and cover the hole on top of the bottle, poke holes in it, pack weed in, hold a lighter over the weed, slowly pull the bottle upwards untill it's almost out of the water in the pitcher (not completely or you'll lose smoke), take the foil bowl off, put your mouth over the hole and inhale as you let the bottle fall back down into the pitcher which forces all the smoke to go into your lungs getting you extra high.