How does a gravity bong work?

Does anyone care to explain? I've never used one or seen one so I'm not sure at all.

 

just suction. you cut the bottom off of a 2 liter and make a bowl where the spout is. You can just use tin foil and poke holes in it for the bowl. THen you submerge the bottle into a tub of water (or your sink) so that its a little over halfway deep in water. THen you light a flame over the bowl and slowly pull up, and this create suction which draws the flame into the bowl lighting the bud. dont pull the bottle out of the water though cause the smoke will escape. then you hold the bottle steady remove the bowl put ur mouth over the spout, push the bottle back into the water and the smoke inside will shoot out into your mouth; inhale

 

you just gotta imagine the inside of the bottle as acting exactly the same as one of your own lungs breathing in from a bowl thats in your windpipe.

 

Magic