No need for poos or farts: Andy Griffiths and Terry Denton are moving out of the treehouse

And in the 12 years that Andy Griffiths and Terry Denton have been collaborating on the Treehouse series of books, there hasn't been an idea that they both thought was so ridiculous that it was never given a storey of its own.

That's a big call, given there are now 169 stories, 13 in each of the 13 books. The treehouse won't reach storey 170 because they've decided that the symmetry of 13 is the perfect place to finish.

It all began innocently enough.

Hi, my name is Andy. This is my friend Terry. We live in a tree.

In that first book there was a bowling alley, a see-through swimming pool, a tank full of man-eating sharks. Pretty standard stuff really.

Over the years they added in an ice-cream parlour with 78 flavours run by an ice cream serving robot called Edward Scooperhands; an active volcano; a snakes and ladders game using real snakes and real ladders; even a toffee-apple orchard. The stuff of imagination.

"The closest we ever got was when I suggested the never-ending staircase, I think it was in 104," says Griffiths.

"I told Terry we needed a whole chapter where they had to climb the never-ending staircase and he looked at me and came back with 25 pages of them climbing, where the only text was 'up and up and up', climbing in all directions with lots of little jokes. There was nothing he wasn't able to draw."

Except in this latest adventure perhaps, where there's a gecko chamber. It started off as an echo chamber.

"He was having trouble drawing it in a way that would make the idea work and I joked maybe we should call it a gecko chamber, and it worked so we kept it," Griffiths said.

Griffiths and Denton's relationship stretches back to the early 1990s, when they worked on a couple of educational textbooks called Swinging on the Clothesline and Rubbish Bins in Space, about livening up creative writing in the classroom.

"Terry was the freelance illustrator assigned to those books and it soon became obvious to me that he had a window into my brain and he could find that weird nexus between the everyday and the fantastic. For him, I think I gave him material that allowed him to unleash his inner anarchist," Griffiths said.

When Griffiths went to write Just Tricking, his first "official book", as he calls it, in 1997, he went straight to Denton for the illustrations.

"As that Just series went along, I realised more and more what Terry was capable of and I said to myself, I have to get out of the way, I need to have less words and more pictures," he said.

"So that began the active collaboration and it's really been a dream come true."

There's also a third unsung member of the collaboration Griffiths wants to acknowledge - his wife Jill.

"She's my wife, editor, co-writer, rhymer extraordinaire. She's the one who keeps us in line," he says.

"If Terry and I have gone too far, in terms of silliness, or the plot has degenerated into pure nonsense, Jill will call us on that.

"The art of it is to not let one voice dominate, it's about keeping those three readers happy."

But there's another reader Griffiths is always mindful of too. Earlier in the year, he wrote a think piece about the controversy surrounding the censorship of classic children's books.

"I'm not surprised - or particularly disturbed - by the fact that it has been deemed necessary to make a number of changes to the words in classic books like The Witches and Charlie and The Chocolate Factory," he wrote.

"Words like 'fat' and 'mad' and descriptions of women as 'fearfully ugly' and having 'horse-shaped faces' might have been - and clearly were - perfectly acceptable in the second half of the 20th century when they were written, but thankfully they have been recognised as offensive, and called out as such."

He says, now, you always have to remember you are writing for an audience whose role as the fourth member of this collaboration is getting bigger all the time.

"What an audience can handle and understand affects how you pitch the story and what you write," he says.

"As we've gone on, over 13 years, that changes, what's acceptable changes.

"Back in the '90s, when we were doing the Just books, there was less scrutiny over what we were. There was a bit, but you could get away with characters calling each other idiot and stupid and telling each other to shut up.

"Nowadays people are a little more uncomfortable with that language, adults in particular.

"We've always taken our audience, not only the kids, but adults, the parents, librarians, teachers, into consideration."

Isn't this a bit rich coming from a man who also wrote The Day My Bum Went Psycho, Zombie Bums from Uranus and Bumageddon: The Final Pongflict?

"The Bum books started off as a bit of a joke, the kids would hassle me about the next book and one day I told them I was writing a serious book next, about the day my bum went psycho," he says.

"I mentioned it to a journalist and she reported on it, quite straight-faced on the regional news, and I thought, this is too good, I have to write it now."

Most people, myself included, are surprised to learn that the word "poo" never appears in The Day My Bum Went Psycho, and the word "fart" only once.

"We found that if you implied the word fart or found a million different ways of describing poo, you're leaving room for the audience to play the game as well, so it makes it more satisfying and more able to be read by squeamish adults," Griffiths says.

He comes back to the idea of leaving room for readers to use their imagination several times during our conversation. Ways in which words, and illustrations, can take readers beyond the page.

"That's what books did for me as a kid. Enid Blyton, Dr Seuss, Lewis Carroll, you'd be enjoying these stories and going, wow, all the things they're making me imagine, there is no limit, it's astonishing, the freedom it gave you to indulge in your own imaginative fantasies," he says.

"That's very much what I wanted to capture when I started writing, I wanted to make sure subsequent generations of kids had access to that type of book."

Griffiths spent four or so years teaching high school English and it was there he noticed kids were neglecting books.

"But books were neglecting kids too," he says.

"It was the mid-to-late 1980s and books kind of lost sight of themselves, they had to have a message, they had to be studied, they were old-fashioned.

"Kids wanted something fun that had no limits, they loved things like the movies, computer games, shows like The Simpsons."

Today, he loves nothing more than doing workshops in schools, or meeting young readers, and listening to their ideas. In each Treehouse book, there's at least one level that's come from an idea of a reader; the most famous is probably the ninja snail training academy.

"That was better than anything I could have made up in a million years," he says.

I think you just need to be patient when searching for the book that will appeal to your particular child. All children, and adults, are different. You need to just relax your ideas of what they "should" be reading, whether it should be a classic, or whether it should be fiction.

For some kids, non-fiction is absolutely where it's at - I loved The Guinness Book of Records as a kid. If they can hook into things like that, they reach for it naturally. But if they sense that you're disapproving of it, or that you're making reading into a chore, they'll fight you every step of the way.

And it doesn't even have to be a book. See if they want to read comics, or the sports pages, or car manuals, or cereal boxes. Just let them reach for things that capture their imagination.