just what slipknot says. For a nice addition, fill the bottle with ice cubes, and drill a bunch of holes in the chunk you cut off. Invert the bottom piece and wedge it into the bottle. Secure with tape or whatever you wish. Now you have a knife hits ice filter, makes knife hits much more bearable, especially if you're smoking resin.

*Warning* if you plan to smoke resin with knives, it will not stick to a hot knife the way a ball of weed will, you have to drop it onto the knife, which can be tricky.

Peas out