Making a gravity bong isn't that hard to make. You first need a 2 liter soda bottle. Cut the bottom part off of the bottle (right below the label.) Then find a pen that has a meatal head to it. Take it off the pen, and then make a hole in the soda bottle's cap just big enough so the skinny end of the pencil head fits in. Use a hot glue gun to seal it in place. See, you have a little bowl head on the cap.
Okay, now that you have made the gravity bong, you need to either fill up a sink with water, or just use a bucket of some sort. Put the bottle into the water, going as far as much smoke you are going to want to have. Then put the cap onto the bottle, and spark it up.
Slowy bring the bottle up, watching all of the smoke go into the bottle. But don' go to far up, for you could loose all of the smoke, and that wouldn't be good. When it looks like a big mushroom cloud of smoke is in the bottle, un-screw the cap, and suck in all of the smoke, by the same time pushin down the bottle back down into the water. You see, the water forces the smoke to go into your lungs, thus gettin a monster hit.