I watched two guys playing darts in a pub. They first put '501' at the top of the blackboard. Then Player A threw his three darts and had to write the three numbers on the board to add them up. He then tried to take that total way from 501 but kept getting stuck, and his mate couldn't help him either, so they eventually decided that it would be easier to count upwards towards 501, rather than counting downwards from it.
I was finding it all incredibly amusing until they both turned round and I saw their faces. It was only then that I realised that I'd been their maths teacher ;-)