How are some people able to bite, chew and swallow glass? Do they really eat it?

How are some people able to bite, chew and swallow glass? Do they really eat it?
I saw a chap on the telly eat a pint glass – he bit into it, chewed it and swallowed it and consumed the whole glass. How on earth do they do this? Doesn’t it tear up their mouth and throat. And … how do I put this delicately? – where does it go? Does it pass out of the body in the usual fashion? And surely it hurts when it does?
Ouch! I am wincing just thinking about this.
Please tell me how they do it?
Ten points and a fluffy kiss to the best answer