Convert a bottle

I've done this before but it's really hard to get a clean cut. They usually end up chipping or shattering when you pour water on them.

My brother has made some pretty fantastic ones, though I don't think this is the technique he uses.

>insert joker.jpg< wanna know how i got these scars?

Had one of them I ordered in the mail years ago.Can't remember where I ordered it from.
Made a bunch of glasses with it!

But... why?