What is "gravity bong"

n. A homemade device used to smoke marijuana, consisting of a plastic jug with the bottom cut out and immersed in a vat of water, which is then raised to create a vacuum beneath its bowl of lit marijuana.

A gravity bong is a method of consuming smokable substances such as cannabis. The term describes both a bucket bong and a waterfall bong, since both use air pressure and water to develop smoke.

The bucket bong is made out of two containers, with the larger, open top container filled with water. The smaller has an attached bowl and open bottom, and the smaller is placed into the larger. Once the bowl is lit, the operator must move the small container up, causing a pressure difference. Smoke slowly fills the small jar until the user removes the bowl and inhales the contents. A waterfall bong is made up of only one container. The container must have a bowl and a small hole near the base so the water can drain easily. As the water flows out of the container, air is forced through the bowl and causes the substance to burn and accumulate smoke in the bong.