Iv witnessed one being done, my mrs done it with some of our friends i wouldnt do it though but i can honestly say they wasnt taking the piss becuse one of the girls does it regular to speak to her little brother who died when he was under 5 years of age and you could just tell the way that glass moved around they wasnt messing about - it also at one point flew off the table and smashed into the wall!!!

wouldnt mess about with them things personally.