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A smoking Aparatus created by myself late one night when bud was scarce, and an efficient, hard hitting device was needed to make the most of the weed we had. It is a close cousin to the gravity bong in the sense that it makes use of the properties of a vacuum, yet requires less work and is more portable. This device is capable of producing a large hit with little to no loss of ANY smoke.

Materials needed:
One 2-liter plastic bottle*
One large ziplock (plastic) bag, big enough to fit around a 2 liter coke bottle.
Tin foil or a bowl for a bong (slide).
Some strong tape, duct tape or the plastic shiny brown tape works well.
A thumbtack or pin.
A knife, scissors, or something of that matter.

*(any size plastic bottle will do, but adjust plastic bag size to suit)

First, cut off the bottom quarter of the 2 liter bottle, right around where the label would end. Then, cut the ziplock off of the bag (if it has one) right where it ends, so you still have the majority of the bag. Firmly tape the plastic bag around the newly cut open bottom of the 2-liter. tape it around REAL tight, this is very important, the seal must be airtight. The length of the bag that hangs off of the bottom of the 2 liter should be just enough to fill the bottle if you were to suck on the top of the bottle. hold it tight and move the bag around to find the best space for it. place a piece of tape in The center of the bottom of the bag, so it is stuck to the front and back of the bag, with some excess hanging off the bottom (this is the handle). Next, take the cap of the bottle and but a hole in it that is .5 to .75 of an inch, if you are using a bowl, make it big enough to just squeeze through, if there is extra space, tape it or use gum. If you have only tin foil, first make a cylinder of tin foil with the center completely open that is the same size as the hole in the cap, and about 2 inches long. Stick this halfway through the cap, then pull all the tin foil coming out the top flat on the top of the cap to cover it as much as possible. this is to cover the plastic, so you are not burning plastic into your smoke. Use the tin foil to shape a bowl on top of the cap. The best way to make a bowl is to wrap foil around the cap to make a simple tube going to a larger bowl, completely open at the bottom, then take a small piece of foil and punch as many holes in it as you can while still keeping it movable. push this into the bowl and now you have a nice screen.
To use this, all you do is take off the cap, then inhale to pull the bag into the bottle. pack the bowl, screw the cap on (with the bowl) light the bowl and pull the tab down nice and slowly, the bottle will fill with smoke. once the bag is pulled down all the way, take off the cap, and either inhale real hard or just pushg the bag in and let it force its way down.
You only need to pack small bowls with this bad boy, and it will wreck you up. Smaller ones can be made with 20oz bottles for portability. They get nasty after a week or so of use, but they are cheap, and you can re-use the bowl.
Good luck