A math joke from The Curious Incident of the Dog in the Night-Time by Mark Haddon:
"There are three men on a train. One of them is an economist and one of them is a logician and one of them is a mathematician.
And they have just crossed the border into Scotland (I don't know why they are going to Scotland) and they see a brown cow standing in a field from the window of the train (and the cow is standing parallel to the train).
And the economist says, 'Look, the cows in Scotland are brown.' And the logician says, 'No. There are cows in Scotland of which at least one is brown.' And the mathematician says, 'No. There is at least one cow in Scotland, of which one side appears to be brown.'
Never argue with a mathematician!Link