Either a homemade or store bought water bong that effectively conserves weed while providing a clear and intense high. Consists of a large, deep container capable of holding water, filled with cold water (example: bucket, beverage cooler, cooler, bathtub. For miniature gravs: large drink cup, larger empty food tins) and a smaller open-bottomed liquid capable container (example: 2-liter bottle with bottom cut off, Large sports-drink bottle with bottom cut off. mini grav: bottomless water bottle, etc). This smaller container is placed inside the initial larger container, with the water level rising to only a few inches below the mouthpiece of the bottle. The grav top is typically the screw-on top of the bottle with a hole cut out in the center and a small socket melted into the hole. The smoked substance (which is hopefully some lusicious heddies) is placed in the metal socket with a flame lit to it. As the user pulls the bottle upward out of the water very slowly, while holding the flame to the bud, the smoke is sucked downward from the socket, into the bottle, cooling slightly as it hits the water. The chamber should, if made correctly, fill with smoke as water is also pulled upwards in the bottle due to the suction. When the contents of the socket are burned to ash, the user unscrews the top of the bottle, and places their mouth to it whilst simulaneously slowly allowing the bottle to fall to the bottom of the bucket again, and inhales the delicious smoke as it is forced out the top mouthpiece by the gravity of the falling bottle as the water level rises.
Gravitron manufactures all-glass gravity bongs that are as effective as a homemade one but are easily broken, not easily stored or transported, and considered legitimate paraphernalia. A bucket and a cut up bottle could easily be some silly science experiment a homemade musical instrument. Just hide the grav top :]
"I hope this article makes sense- I just pulled a grav."
"Shit, I just pulled 4 gravs. I'm BLAZED"
"Let's go make a grav."