here is a movie of how to do it.......good luck
http://www.waytoomany.com/vids/weed/gravity_bong.php

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Fez4Prez Reviewed by Fez4Prez on . The new Hottness... Waterfalls Gravitys are good and all, but I've recently built a waterfall bong and it is the SHIT! We take one hit each and we were all like 'I'm done'. And we were baked. I took a slide from any normal glass bong and put it through a hole I made in the top of a cap that fit it just right and the slide has a little rubber ring around it at the base of the bowl that helps keep the seal further. Screw it onto any plastic bottle and punch a small hole in the bottom about the size of a writing pen. Rating: 5