Child-Beating

It's educational, it costs nothing, you can learn it in seconds and - best of all - it's a game at which I can beat my children. I cannot remember where I first met the game I am about to describe, but it suddenly came into my mind last week and I challenged my kids to a game. They drew a few times, and they lost a few times, and they gained some good practice at elementary arithmetic, but they never won (except for once, when I had a perfectly good excuse which for the moment eludes me).

Here's how it goes: you select nine cards from a pack bearing the numbers 1,2 ... up to 9 - or you can just write the numbers 1 to 9 on nine small pieces of paper. It's a game for two players, starting with one of them picking one of the cards. All are turned face upwards, so you can choose whichever number you want. The other player then picks one of the eight remaining cards, and the players continue selecting cards in turn. The object is to build up exactly three cards that add up to 15. The trick of the game is to realise that what you are doing is playing blindfold noughts-and-crosses, as may be seen by examining the numbers in this three- by-three array: Every three-number sum to 15 is a winning noughts-and- crosses line. So if the first player begins with 5, the second must reply 4,8,2, or 6. If the first starts with a corner number, only 5 will save the game. Easy when you know how. But don't tell the children.