Doing some ironing this morning I was listening to a play on the radio. The plot was based on a group of friends who were solving clues to work their way out of a locked room. One of the clues asked for the value of the product of all the digits on a standard western telephone dial. The characters in the play immediately set to work multiplying 1×2, then that product by 3, and so on. The maths didn’t last for long, as the numbers grew larger and the ensuing mental arithmetic more demanding. They were quite quickly interrupted by the female member of the cast, however, who pointed out that all of the telephones she had come across in her life had the number zero on the dial, and that any number multiplied by zero is zero so they needn’t concern themselves with the tricky sums! A lesson to us all: think before you leap into mental arithmetic – or many other things for that matter.