you havent hit a gravity bong until it been filtered by the water! its the best hit in the world. hands down. then put a bunch of ice in the bottle, it will make your body temp drop its so cold!
the only thing you need to know is that it has to be super sealed up and the stem cant be longer than the bottom of the bottle. see the red line on the bottle? thats where it bubbles.

naw dude i just recently found out that this would work, been a theory for a long time but never tryed it until seeing a vid on youtube.