It has happened to most infant teachers. It’s “showing” day in Year 2 and along with the pictures made of pasta, the shells found on the beach and the bowl of frogspawn is something wrapped in a bloodstained tissue.

It’s a milk tooth, proudly presented by the six-year-old who also wants to tell you what the blood tastes like when he sticks his tongue into the hole in his gum. But this exhibit won’t stay on the showing table. It will go home for the tooth fairy.

Tooth fairies are the goods. They don’t simply flit around idly drinking out of acorn cups. They dole out hard cash. Like fairy godmothers, who are strong on transport and clothes, these are little people worth believing in. Even if you don’t really.

“The going rate now seems to be a pound, occasionally 50 pence, a tooth, ” says Mrs Yvonne Smith, who teaches gap-toothed Year 3 infants at Summercroft Infants School in Bishop’s Stortford.

A pound? With 20 milk teeth per capita, the coffers in fairyland must be heading for bankruptcy, unless the fairy in author Philip Hawthorn’s story, The Tooth Fairy, is to be believed.

She tells young Alice, the heroine of this rather alternative fairy story, who keeps disgusting things in matchboxes, that each milk tooth is changed by magic into a pearl. In which case, it’s foreign worker exploitation on an undreamt of scale.

At Summercroft, a wobbly tooth is all part of number work. Watched over by a big cardboard fairy, there is a bar chart on the Year 3 wall, with the words “Who is she going to visit next?” “Each child fills a section of the chart when he or she loses a tooth,” says Mrs Smith. “It just struck me that it was quite an exciting thing in their lives which we could use for maths practice. ”

Not to mention biology. By the age of two-and-a-half we have all our so-called milk teeth, which we start shedding around the age of six. They’re replaced by a full set of 32 adult teeth, including four wisdom teeth. But just as those four extra molars you get around the age of 20 don’t make you wiser, your milk teeth have very little to do with milk.

They are, as a British Dental Association adviser explained, much the same as adult teeth. They have crowns and roots and they do the same job, tackling for several years much the same food we eat as adults. So why do we - and all other mammals except whales - lose our first teeth?

“It seems to be something to do with size,” the BDA adviser said. “Full-size teeth wouldn’t fit in an infant mouth. Teeth don’t grow with us, like bones, so we have to lose them and get some which fit.”

All very rational, but teeth are the subject of a lot of superstition. The child who puts his tooth under his pillow is doing what generations before him have done.

“But why,” asked my mother (sixpence was the going rate when she was bringing her children up), “did we put salt on them when I was a child?” According to A Dictionary of Superstitions, edited by Iona Opie and Moira Tatem, the connection between salt and milk teeth is very old. It quotes a 17th-century source on the custom of salting the tooth before burning it in a fire. More recent references listed include salting it and throwing it over your left shoulder for luck.

A quick cross-reference to salt reveals a belief in it as a luck-bringer and a protection against evil that stretches back to the Bible and even to Homer. The tooth fairy seems a rather more recent and much merrier creation. Perhaps she came in with central heating and the end of the coal fire.

Who wouldn’t rather risk a bit of blood on the pillow case than having to burn a tooth with salt in case it was found by a dog and the child grew dog’s fangs instead?

The squeamish can even buy little silver tooth fairy boxes from catalogues and High Street jewellers - just the right size for a pound coin - so there’s no gore on the linen. How comfortable they are to sleep on is another matter.

We’ve always been funny about teeth, especially about children born with them. In Shakespeare’s Richard III, the murderous king is “That dog, that had his teeth before his eyes, To worry lambs and lap their gentle blood.”

Opie and Tatem say superstitions about new babies with teeth range from exceptional cleverness to being born to be hanged. Dreaming about tooth loss was thought to mean a death.

But the tooth fairy hasn’t taken over everywhere. One Hertfordshire family follows a tradition peculiar to their thatched house. They are at least the third family to push their children’s milk teeth into the crevices of the vast chimney.

Children are fascinated by the idea that a bit of themselves has painlessly dropped out. Wobbly teeth are wiggled for fascinated friends. Roots and gore are appreciatively examined by youngsters who have an eye for a tooth.

When some newly-shed pony teeth were brought into Class Two of Ditchling Church of England Primary School in Sussex the other week for “showing” day, the five to seven-year-olds assessed them with expert eyes.

“Massive, but just three roots,” said my daughter Rosie, aged five, when she got home. “And that’s because one of the roots is still stuck in the pony’s gum. Stuck! I wish I could lose a tooth.”

I wish I could stick it up the chimney when she does, but the going rate is a pound and I can hear the beat of fairy wings.

Oliver’s Sundew Tooth Fairy by Sam McBratney (Walker Books Pounds 3.50); The Tooth Fairy by Peter Collington (Jonathan Cape Pounds 9.99); The Usborne Book of Fairy Stories by Philip Hawthorn, illustrated by Stephen Cartwright (Usborne Pounds 6.99); The Oxford Reading Tree includes a Chips and Kipper title called The Wobbly Tooth.