As cannabis is not water soluble and nicotine (+ many other none cannabinoids) is it is advisable to always smoke cannabis by pulling it through water. In many situations this is not available (prison , outdoors etc) and the user has to turn to something like a Lung to get the job done.
Usually a lung is made from a plastic bottle 2lt or larger with the bottom cut off and a plastic bag taped taped to the bottom of the bottle in its place. If the bag is airtight it is pushed upwards inside the bottle and a head (13ml socket set + gauze) is placed into the top of the bottle then filled with whatever the user is smoking.
The inflation of the lung is usually a co-operative process involving 2-3 people as the bottle needs to be held upright to stop the head from falling out while the bag is pulled from the bottom into an inflated position while a lighter is held over the top of the head to light the smoking material. The action of manually inflating the lung draws large amounts of air into the the plastic bottle filling the plastic bag. The smoker then has to remove the head (ouch its hot >.< ) and attempt to suck the entire contents of the bag into their lungs.
(though it can be operated by a single person by attaching the bottom of the bag to the floor or by standing on it)
Because of the the amount of smoke inside the bag is very large the user has to breath in very deeply to empty the bag in one go. This has the added effect of drawing smoke into parts of the lungs that normal smoking doesnt necessarily reach which results in a much higher level of THC absorption that a user is may be used to. Also the deep breathing can cause a certain amount of hyperventilation raising the users blood pressure making them somewhat dizzy. Put those together with the fact that most people using a lung are 'smoking noobs' and have a very low tolerance to THC in the first place the end result is usually an extremely potent high.
I have no idea what breathing the burning plastic made by the hot ashes from the head hitting the inside of the plastic bag as its inflated does to your lungs O_o.
"Fuck it! well have to make a lung instead i guess"
"A plastic bag, some tape and...the bong...."
"Fuck you asshole!! your not cutting the bottom off my bong!"
"well how else we gonna get fucked up Einstein?"
".....fuck it! OK... but your not using my moms scissors she'll kill me.."
*cut* *snip* *cut*
"Jesus dude whats that smell? When was the last time you cleaned out your bong!??..."
"errr...when i made it? Shit! don't put it on the carpet you idiot my mom will fucking freak!"