Did you hear about the constipated mathematician? He worked it out with a pencil.

True one: I worked in a school where the Headmaster took assembly every morning and after the religious bit he did his daily rant about behaviour. On the day after a window had been broken by a cricket game he went on and on and on about not playing with your balls near the building, and if you must play with them make sure you stayed on the tennis courts. And 'Why are you sniggering boy? It's causing too much damage and that's no laughing matter'.

There were no staff left in the hall by the time he had finished!