Take a large onion and chop finely. Place the pieces in a mediumsized casserole dish. Now mix ten tablespoons of washing-up liquid with a tablespoon of salt, and make up to two pints with water. Add about a quarter of this mixture to the onion and cook in a bain-marie in a very cool oven for five minutes, stirring frequently, and liquidise at high speed for just five seconds.

Now strain the mixture and add a few drops of fresh pineapple juice to the strained liquid, mixing well. Pour into a long chilled glass and finish off by dribbling ice-cold alcohol (vodka will do) down the side so that it floats on top of the mixture. Wait a few minutes and watch cloudiness form where the two layers meet. Now lower a swizzle stick into the cocktail and carefully hook up the cloudy material. It should collapse into a web of fibres that you can pull out of the glass. This is DNA (short for deoxyribonucleic acid).

DNA is the stuff that genes are made of. Genes carry biological information, which is translated into the characteristics of living things and is passed on down the generations. So genes determine the colour of a butterfly's wings, the scent of a rose, and the sex of a baby. DNA is just a chemical – not a more complex entity like a chromosome or a cell – and it is only in a biological context that it acquires its status as the molecular signature of an organism.