Ok let me throw some insight on how to really hit a gravity bong and the science behind how it works. First off a gravity bong uses water pressure to pull heat into the bowl as your toasting it. You can easily snap a bowl if you dont watch it, which in turn will put you into another D. Ok so the first step in making one of these goes with whatever jug, bottle you like. Remove to bottom( i like to have about a little more than 2/3rds left. With the cap make a pinki size hole. Put a slider in the hole, or a socket, or a pen tube, or just a foil bowl. That depends on what you like to use. Submerdge the bottle before you have the cap on, unless you want wet weed. You then place the cap which has your weed on. Light the bowl and as your lighting pull the bottle up. When the amount of smoke you like is in you remove the cap place mouth on and push the bottle into the water. This is the best way as the smoke will get deep into your respiratory system. This is better than sucking in the smoke. For a quick anatomy lesson. When smoke is forced rather than sucked it gets deep into your lungs, hell it gets in your respiratory system. This just makes the high crazy. Good luck and remember to have fun.
I use a 3 litter bottle or a 20 fl oz bottle, this is depending on how messed i want to get. Remember the larger volume of a bottle the more air is going to pulled from the bowl generating heat.... i.e- Those hits were massive from the gravity bong.