Gravity bongs are easy to make.

Get a bucket or a big tall pot and fill it with water.
Find a milk jug or big soda bottle or something.
Cut off the bottom of whatever you found,
and either the bottles neck is small enough to simply place a bowl peice on top,
or its slightly bigger and all you need to do is cut up the cap so that the bowl peice can slide in nicely.

Put the jug/bottle/w/e in the water, screw the cap/bowl peice on, put small amount of weed in.
Now, you want to slowly pull it out of the water while lighting the weed. Slowly. Once it's lit, keep going, but make sure to stop pulling before the bottom comes out of the water. Then simply be careful with it, try not to jiggle it at all or anything while you unscrew the cap, wrap your lips around the top, and push down on the jug/bottle/w/e semi-fast while inhaling. And you want to exhale as much as you can right before you do the inhaling.
Then brace against the FiYah!