Why do we laugh when someone falls over?

Why is it funny when people fall over? What are jokes for? A session for teenagers at Cambridge University today will come up with some answers. A man is walking down the street gazing into space. He doesn't notice an open manhole, and disappears into it. It's hard not to laugh at a silly scene like this - but why we find it so amusing is an interesting question. The philosopher Henri Bergson argued that laughter helps to maintain the rules of society. We learn to laugh at careless or eccentric behaviour, and this makes people pay attention to the requirements of the world they live in, because nobody likes being laughed at. Laughter pushes us towards being normal.