Horses and anecdotes: Suddenly you get aware there is "a pair of horses", and not just one - that is, not just one set of associations and thereby not just one frame of reference. Such a sudden perception ("bisociation") on the funny side of thinghs makes for very much humour, explains Arthur Koestler in his book The Act of Creation (Penguin, 1989:35-46. etc.). See also his humour article in Encyclopaedia Britannica (s.v. "Humour", and Wikipedia, s.v. "The Act of Creation".