According to Sigmund Freud, jokes represent the return of suppressed thoughts. This may explain why mortuary attendants notoriously have the filthiest sense of humour - you can imagine a lot of suppression going on round the morgue. But does it explain why children think knickers are so funny? After all, they are more prone to showing their underclothes than most adults.
Perhaps the political explanation, that jokes are a way for the oppressed to subvert tyranny (the Czech underground used to be famed for its humour) is more useful for understanding children. It is deeply gratifying to see authority figures with egg on their faces, but you don't have to be a child watching Roald Dahl's Matilda for that. I had the same thrill of comic release watching Michael Portillo lose his parliamentary seat on May 2. Still, this does not seem to explain why "Knock, knock, who's there?" "Isabel." "Isabel who?" "Is a bell really necessary on a bicycle?" is funny. Nor does the Freudian notion of a repetition compulsion seem adequate to explain why it is - to judge by observing a joke session in the playground - funnier the fourth time around.
Children's jokes, like their laughter, are playful. Drawing life in primary colours, children enjoy the drama of bodily functions and the grotesqueries of adult behaviour; it's no accident that Men Behaving Badly is the favourite TV programme of many older junior-school boys. Girls, on the other hand, may often prefer Mr Bean and gentler social comedy such as The Vicar of Dibley. All very physical. But it's pantomime, with its broad characterisations, ribald look at sexual relations, talking cats and cows and horses, deliciously horrid villains and singalong choruses with silly gestures that best epitomises a child's eye-view of comedy. Nothing too subtle, please. Belly laughs are better fun.
Let's not get too sentimental, though. Laughter at difference, people who "walk funny" or "look funny" can be hostile. "Teasing" is all too often a way of distancing and making the other distastefully inferior, reducing what is seen as a threat to identity. It's not by accident that we call this jeering "making a butt" of someone. Laughing at people who are excluded makes those in a group feel solid and powerful. This is the joke as weapon - Jim Davidson or Bernard Manning humour. It's not "just fun, miss".
Laughter can wear other faces, too. Hectic, hysterical giggling, as many a parent knows, can mean "tears before nightfall". According to psychoanalyst D W Winnicott, this kind of laughter springs from the over-excitement of immature organisms who cannot find release through orgasm. Ho hum. Don't laugh. In fact, take that smile off your face, child. Don't you just hate that, when they laugh behind your back?
The funniest thing I ever saw was the late Arthur Askey as a pantomime dame taking off his voluminous underclothes. There were 15 (numbered) pairs in different colours.
He took them off very slowly and people (not just children) started falling off their seats laughing. The last pair of padded frillies stayed firmly on, with a notice plastered across Askey's rear reading: The End. People were clutching their sides and moaning. Did the earth move or was it just the Golders Green Hippodrome?
Back in the classroom, our resident sage offers this gem. "The funniest thing is sex. Or bottoms. And the other funniest thing is: What is white and doesn't fly. Answer: A fridge."