In his wonderful book, The Tipping Point, Malcolm Gladwell analyses how ideas and trends suddenly cross a threshold and "tip" into epidemics.
Particularly intriguing was the story of how a rather slow and unpreposessing pre-school TV programme called Blue's Clues became more popular than jazzy old favourite Sesame Street with American under-fives.
The new show did it by paying absolutely literal attention to the way little children learn, and what holds their interest. Three key aspects were repetition, audience participation and narrative. Unlike the much-loved Sesame Street, which is filled with advert-like short segments, Blue's Clues - which features a real man named Steve and a cartoon puppy called Blue - tells a story over its 22 minutes. This works, says psychologist Jerome Bruner, because narrative is the way children organise and understand the world.
Blue's Clues doesn't have jokes or sophisticated puns above their heads.
Everything is literal. In each episode the audience is presented with a puzzle, and Blue leaves clues of increasing difficulty. Steve talks directly to camera, but in an unusual way. "Whenever he asks a question, he pauses. But it's not a normal pause. It's a preschooler's pause, several beats longer than any adult would ever wait for an answer", explains Gladwell.
The most radical thing about Blue's Clues is that the same episode is shown every day for a week. We all know how kids like to watch things over and over, but it took a real leap of imagination to broadcast a programme in this way.
The programme-makers at Nickelodeon found that pre-schoolers' interest increased over the five viewings, and of course they were able to answer the questions more successfully every day. Very young children, they reasoned, want predictability, more than novelty, as they are surrounded by things they don't understand.
One of the developers told Gladwell, "When they see a show over and over again, they not only are understanding it better, which is a form of power, but just by predicting what is going to happen, I think they feel a real sense of affirmation and self-worth".
Write to Primary@tes.co.uk "Blue's Clues" has appeared in the UK on Channel 4, and can be found on the digital TV station Nickelodeon. "The Tipping Point" by Malcolm Gladwell, Abacus, pound;7.99