What is a gravity bong?

a gravity bong is a type of bong that is usually home made. First, finish your 2 liter soda bottle and cut the bottom of the bottle. Second, make a hole in the middle of your cap and put a bowl on it, make sure it is the good size. Third, fill a bucket with water. Forth, put the bottle in the water leaving the cap sticking out of the water. Fifth, light your weed up and enjoy.

If you want pictures, go to google images and type gravity bong. I guarantee you will understand what it is and what it is for.