Instructions:

  1. Get two 2-liter bottles, a screen, a sharp knife, a bucket / largish water receptacle, some tape (electrical, duct, or masking is best), and a drilling impliment
  2. Take the caps off of the bottles. Drill a decent-sized hole through the tops of the caps, put the tops together with the screen in between, and tape together. the result looks like a female-female connecter with a filter
  3. With the knife, cut the bottom off of one of the bottles.
  4. Screw the cap contraption into the bottle you just cut, and discard the shit that's lying around. Put away the knives and drilling impliments.
  5. Put the material in the cap thing (notice how it's shaped like a bowl? cool, no?)
  6. Slowly immerse the bottle in water. Not too fast, or you'll volcano the material
  7. Light the material, then slowly draw the bottle up. The chamber of the bottle will fill with really thick smoke if you're doing it properly.
  8. Keep the bottom of the bottle in the water; you don't want the smoke to escape.
  9. Remove the cap, put your mouth over the top of the bottle, and push the bottle back down into the water. Remember to breathe in :)
  10. And try not to fall backwards when you're done...
If you're good with a drill, try to make the hole the same size as the glass tube of your bong. Then you can use that rather than the second cap. Slightly higher class.
My gravity bong is slightly different than flamingweasel's, so I'll describe mine here

Ingredients/Supplies:
  • One 600 ml pop bottle. (larger if you like, I suppose, this is big enough for me)
  • Bowl assembly of some sort (I use one I bought from a local head shop)
  • Knife, or any suitably sharp object
Procedure:
  • Cut a small hole in the bottom of the bottle
  • Cut a hole in the cap to put your bowl in
  • Put your bowl in the cap-hole
  • Fill bowl with your favorite cumbustable, plug the bottom-hole with your finger, fill the bottle with water, put the cap/bowl on, light, and release the water.
  • enjoy
According to Ground Control's definition there, I am a Certified Bong Technician.

Note:I've tried gravity bongs of the sort described by donfreenut, and they are highly (no pun intended) recommended.

flamingweasel's write up gives a good account of how to make a gravity bong (aka "bucket" in the UK) out of two plastic bottles. However, when it comes to drug paraphernalia, diagrams are always useful in understanding how the device works. Apologies in advance for my ASCII art skills!

N.B. My design differs slightly in that it uses a nut (bowl) and downtube from a conventional bong, rather than the handmade bottle-top bowl flamingweasel uses.


Key:

####
####    =  unburnt "mix" (i.e. roasted tobacco and cannabis)

$$$$
$$$$    =  burning "mix" (i.e. you're holding a lighter under it)

~~~~    =  water level

****
****    =  smoke

|  |
\  /
 ||     =  "nut" (UK) or "bowl" (USA)

[=  =]
[=  =]  =  bottle top with hole in centre

           |##|
           \##/
            ||
          [=||=]            STARTING
          [=||=]            POSITION
          | || |     (top bottle-half is down)
          / || \
         /  ||  \
        /        \
       /          \
      /            \
     /              \
    /                \
|  |                  |  |
|~~|~~~~~~~~~~~~~~~~~~|~~|
|  |                  |  |
|  |                  |  |
|  |                  |  |
|                        |
\________________________/

           |  |
           \$$/
            ||
          [=||=]            BURNING
          [=||=]            POSITION
          |*||*|     (top bottle-half pulled up)
          /*||*\
         /**||**\
        /********\
       /**********\
      /************\
     /**************\
    /****************\
   |******************|
   |******************|
|  |******************|  |
|~~|~~~~~~~~~~~~~~~~~~|~~| <- N.B. water level will
|  |                  |  |    take a second or two
|                        |    to stabilise
|                        |
|                        |
\________________________/

           ****  <- smoke going into mouth
          |****|
          /****\            INHALING
         /******\           POSITION
        /********\     (cap unscrewed and top
       /**********\     bottle-half pushed down)
      /            \
     /              \
    /                \
|  |                  |  |
|~~|~~~~~~~~~~~~~~~~~~|~~|
|  |                  |  |
|  |                  |  |
|  |                  |  |
|                        |
\________________________/

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