If you want to see what happens when someone ignores the rules take a look at the encounter between Ricky Gervais and Elmo which you can find on YouTube.
It begins with the comedian establishing his own version of the puppet etiquette - no talk about drugs, child abuse or the Holocaust.
And it proceeds - via a mention of necrophilia, surely something of a first on Sesame Street - to a discussion of why Elmo goes around naked all day in his natural red fur but then puts on pyjamas before going to bed.
Elmo takes it all in his stride.
"It's called acting, Mr Gervais. ACTING," he declaims grandly.
The journalists who are being invited to meet Elmo to mark the programme's anniversary probably tend to be a bit less challenging than that - I certainly was.
But of course, while we would tend to stick rigidly to the rules, there is no guarantee that Elmo will.
In fact, in the hands of Kevin Clash, the brilliant puppeteer who is always on hand whenever Elmo appears in public, the rules tend to go out the window.
Elmo is a bright-red bundle of love, curiosity, innocence and energy in the show - and in person he is extraordinarily compelling.
On its 40th anniversary, Matthew Stadlen spends Five Minutes with Sesame Street
He seems so real, for want of a better word, that the producer working on the story with me instinctively pointed the microphone at the puppet's mouth, not at Kevin Clash. Happens all the time apparently.
I gamely applied the rules - as best I could - asking Elmo such age-appropriate questions as the name of his best friend.
Elmo, of course, is bound by no such rules and is thus able to run rings around his interviewers - or at least, this interviewer.
Having established where I lived in the US, Elmo was able to recommend the oysters in a local Cajun restaurant - not quite what you would expect from a three-year-old.
Nor was his explanation of why he does not have eyebrows - a taste for very hot wasabi, it seems.
Kevin Clash - apart from having a mischievous sense of humour - infuses Elmo with life. When you are in the character's company he seems as real as you seem yourself and his co-stars, Rosita and the Telly Monster exude a similar, life-like charisma.
Telly Monster is meant to be only five but in the hands of Martin P Robinson, his human alter ego, he radiates the sort of amused world-weariness you would associate with a stand-up comedian from the days of vaudeville.
Sesame St has adapted to foreign markets with local puppets
Rosita - played by Carmen Osbahr - has a mildly flirtatious quality which I must confess to finding rather attractive.
And if you think that sounds ridiculous, you will just have to take my word for it that when you are in the company of the puppets, it is easy to find yourself captivated by them.
And of course that is precisely what makes Sesame Street such a powerful tool for teaching - not for nothing is it known as the world's greatest informal educator.
Children are spellbound by the characters and are drawn into a world where it seems somehow natural to absorb their letters and numbers as they watch.
In the episode we saw being filmed for example, the letter "I" goes missing - stolen by a dog. Telly and Elmo do eventually get it back of course, but as they do there is plenty of discussion about how the letter is able to make two distinct sounds.
"I" is for ice cream of course, but it also starts the word "iguana" too it is a painless way for children to embark on the path to literacy and numeracy.
And of course, there is more to education than simply learning your numbers and letters. Sesame Street has an explicit social purpose too - lessons about the importance of learning to share, and about tolerance, are part of the mix as well.
Rosita, for example, is an immigrant from Mexico - one of her jobs is to teach the audience a word of Spanish every day - and the manner in which she is accepted by the other characters teaches young Americans an important subliminal message about inclusion and social cohesion.
The Sesame Street formula is powerful and flexible. The format has been exported to about 120 countries where it is fine-tuned to take account of local circumstances.
The South African version, for example, includes a character called Kami who is HIV positive and whose mother died of HIV\AIDS. The use of a puppet in a children's TV series to teach lessons about tolerance and bereavement is proof that Sesame Street is flourishing as it reaches its 40th birthday and still finding new worlds to conquer.
Inevitably, as Sesame Street's anniversary approaches, you will hear lots of facts and figures about the scope and scale of its reach - foreign markets conquered, number of episodes taped, total weeks that its hit songs (Rubber Ducky) have spent on the charts that type of thing.
But the things that really matter cannot be counted - the number of hearts that have been lifted and the number of imaginations that have been fired over the years.
The Sesame Street mixture of slapstick, artfully disguised education and gentle moralising is a robust one and it seems certain to last many more years - continuing to teach the children of the world not just how to count, but how to become the kind of people who can be counted on.
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