Here's how I made my waterfall bong, a variant of the gravity bong. You need a soda bottle (I used a 20 oz Dr Pepper bottle), a pitcher of water, the bowl to a metal pipe, a tack and a pair of scissors or knife.

First, unscrew the soda cap and push the tack into the center of it. Then, open the scissors and put the end of one blade on the tack hole (or put the end of a knife on it) and start twisting, making a circular hole. When it is just a little bit smaller in diameter than the bottom of the bowl to the matal pipe, try to shove the bowl through. With enough force, it should pop through and create a near-airtight seal. If it's not airtight, a little chewing gum can plug up any holes.

Now use the scissors to poke a few holes in the bottom of the soda bottle. Slowly submerge the bottle into the pitcher of water, allowing the water to seep in through the holes. When the bottle is resting on the bottom of the pitcher, screw in the cap & bowl (with weed in it). Now lift the bottle out of the water while applying flame. The water will trickle out of the bottom, drawing in smoke from the bowl. When the water has drained and the bottle is full of smoke, unscrew the cap and inhale it all.