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Exerpt from the FAQ-bong.txt
To be found at 
http://www.hyperreal.com/drugs

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GRAVITY BONGS
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Also known as bucket bongs, beach bongs, and depth charges, this is
essentially a device that uses gravity and air pressure to draw the
smoke into a large chamber and then expel it quickly into the lungs.
This gives much larger hits than most instruments, and it is possible to
get quite fried quickly with a relatively small amount of weed. The most
popular method is to take a 2-litre and a 3-litre bottle. Cut the top
off the 3-litre at the point where it starts to curve into the neck, and
the bottom off the 2-litre. Attach a bowl to the top of the 2-litre (or,
preferably, attach one to the lid so it can be taken off). Fill the
3-litre up with water. Place the 2-litre into the 3-litre and attach the
filled bowl to the top. Then light the bud as you slowly draw the
2-litre up. This will create a vacumn in the 2-litre bottle and suck the
smoke down into the chamber. Once you get near the top, quickly remove
the bowl, expel all the air out of your lungs, put your lips over the
top of the bottle and push it back down quickly. This will force all the
smoke into your lungs quickly.

You can experiment a little bit with this design, using different sized
containers and such, but the model above works as well as any other I've
tried. It's easy to use it in a kitchen sink filled with water as well.
Another popular method is to just put a small hole in the lid instead
of a bowl and placing a lit joint in the hole. Draw the bottle up, and
it's possible to get an entire joint into the bottle to be taken in your
lungs at once.

WATERFALLS
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This is essentially a variant on the gravity. You take a bottle (I use a
2-litre) and drill a small hole (about 3/8") in the bottom, at the lowest
point.
Cover this hole with your finger and fill the bottle up. Then, attach a
filled bowl to the top (I use the same Coke-bottle lid as the gravity) and
light the
dope. Uncover the hole out the bottom, and as the water drains out, the
smoke will be drawn in. Keep the bowl lit and let the water drain out,
and by the time you're done you have a 2-litre bottle full of
concentrated smoke. Just suck from the top and uncover the hole at the
bottom to hoot. This is also an extremely efficient design, as very
little smoke can escape.