for a good gravity bong, you have to make sure the chamber isn't too big...remember, you only have so much room in your lungs. a 2L pop bottle seems to work perfectly...and always brew the hit as slow as you possibly can, to get the harshest hit possible. my friend's parachute was used so often, and brewed so long, that the smoke it produced was actually dark brown...he had to clean it every day, just so the bottle would stay clear enough to see the hit inside! also, it tasted like burning, and made me cough a lot. then I fell asleep, which made me late for class. Good times, good times...