· Sometimes people ask: what are your qualifications, to decide what is and isn't bad science? The answer is that it doesn't take much. Take the new television advert for cleaning product Cillit Bang: "Limescale is simply calcium that sticks, and if solid calcium dissolves this fast [lump of solid calclium starts fizzing], imagine how Cillit Bang works on taps and sinks." Limescale is not calcium that sticks. Limescale is calcium carbonate. Calcium carbonate is a chalky substance rather a lot like, let's say ... chalk. You may have noticed the white cliffs of Dover not fizzing. Whereas calcium is a shiny silver metallic substance that is shiny, silver, and a metal, unlike limescale and chalk. When you throw a lump of calcium into water, like most people did aged 15, it fizzes and dissolves. And all those of you who thought calcium was white and opaque like teeth and bones and milk: isn't it great to peer behind the curtain?

· Of course, some people don't appreciate critical appraisal of their ideas. There isn't enough room in the paper for me to pour sufficient bile on the ludicrous claims of Penta water, so I wasn't going to bother, but now I'm strangely motivated. After last week's piece about their failure to provide compelling evidence for bizarre claims about clustered water (uncritically written up in half a dozen national publications) I started receiving nasty, menacing text messages from them. Imagine this buzzing into your pocket: "Goldmember I do hope you are a better physician than you are a journalist when we publish you will of course be informed out of the decency/courtesy you didn't show to us! Sleep well tonight and think about how and why you tried to fuck us over and practice [sic] keeping one eye open." Needless to say, I've gone to the police. When I say I don't like being threatened, I don't say that to sound tough. I mean, it's not very nice being threatened. In fact, they sent it twice, just so that I could be in no doubt. Then suddenly I started getting calls from PR guru Max Clifford to apologise for a "hotheaded" Penta staff member.

· Now, how often do you reckon a loser science journalist gets a call from Max Clifford PR? Weirdly, I last got one about six months ago, when we were looking into someone you may remember. She was called Dr Gillian McKeith PhD. What could she have in common with Penta water? They have each been the subject of more than five Bad Science columns. But, hang on, Penta's only been in two. So far, that is.

Please send your bad science to bad.science@guardian.co.uk

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