If you are parent whose teens have a party, you may find amongst the bottles and cans a few objects that raise a nostalgic smile. Most people my age will have fond memories of the soda syphon. My grandmother gave me a splendid Edwardian looking one with a glass bottle and metal mesh around it, and these little cartridges of carbon dioxide used to be commonplace before the likes of Sodastream made them untrendy. The chances are, though, that if you find these after a party, they aren't quite what they seem. If you happen to find a box you will discover that they are not CO 2 cartridges, but intended for 'cream chargers' that are used to produce catering quantities of squirty cream. And the gas in the cylinder is not CO 2 but N 2 O - nitrous oxide - commonly known as laughing gas. This is, it seems, the latest party and nightclub thrill. The good news is that the gas is not illegal, and if used properly is less dangerous than many drugs. But it's not all good news. ...