Drain pipe (ABS- it's lighter than PVC), is a fantastic building material- the joints will effectively seal without glue, so modular pieces are easy to make. My favorite creations from college:

• a piece on which one could switch the bowl and the neck, effectively converting it from a bong to a giant bubbler (Edit: pic!- also, at one point I had a fair amount of ABS laying around so my roommate and I extended the neck of this thing about ten feet. I was in the bedroom while he lit it in the hallway. Good times!)

• numerous hand-painted gravity bongs (although the name always bugged me- why not "pressure pieces" or "vacuum bongs"?)- two foot lengths of 1.5" and 2" ABS with rubber test-caps on the appropriate ends (just found the old pics! 1, 2, and 3- I had others=)

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