Have you heard the one about the comedians doing stand-up comedy and sketch shows for children?
Comic James Acaster does what most funnymen would never dare to - he invites the audience to yell insults at him.
One voice tells him "you smell", causing hilarity among the rest of the crowd. Another informs him he is wearing a silly shirt.
This is a different sort of insult to those wielded at most comedy clubs. But this is a different sort of club.
Comedy Club 4 Kids has been tickling children aged between six and 11 - plus mums and dads - at the Edinburgh Fringe for seven years.
Only when one mum lobs in a more fruity insult is the innocence temporarily punctured and the goofy Acaster moves swiftly on to some deliberately rubbish magic tricks.
Until recently, live comedy for children usually meant pantomimes, variety shows or party entertainers.
But there is a growing trend for stand-up comedians to perform two routines a day at the Fringe - one for children in the morning, before a grown-up show later in the day.
Alex Horne, whose music-based comedy has been heard on BBC Radio 4's The Horne Section, tried a slot at Comedy Club 4 Kids last year. He is now back in Edinburgh with dates for families on top of the regular late-night show.
"The children's show is not really different from the grown-up show," he says. "And we know from our experience that children are basically the same as a late-night drunk audience - uninhibited and lively and ideal for what we do."
The most obvious difference between children's comedy and late-night stand-up is that there are no rude words. Apart, perhaps, from bum, poo and fart, which never fail to generate raucous laughter.
Tiernen Douieb, a comedian who runs Comedy Club 4 Kids, says: "There are an awful lot of stand-ups that are, like myself, very childish.
"And we get to have an audience where we can do our silliest, most ridiculous material without any cynicism or groans."
Children's comedy shows are indeed very silly, innocent and free of the confrontation that can arise in normal stand-up clubs.
"Kids are a really enthusiastic audience," Douieb says. "They race to fill the front row first, which adults often don't do, and they rarely heckle maliciously.
"They just want to contribute and be part of it and they're very excited and that's a real joy to perform in front of."
Douieb, who has staged both adult and children's shows at Edinburgh, says the crowds have often been bigger for the youngsters' versions.
"The only money I've ever made in Edinburgh, which wasn't very much, was for a kids' show I did a couple of years ago, which sold out pretty much every day," he says.
There is no longer any shame in performing comedy for children. Last year's winner of the Fringe's prestigious Fosters Comedy Award, US clowning comic Doctor Brown, also does a children's show titled Dr Brown Brown Brown Brown Brown and His Singing Tiger.
The rise of live children's comedy has coincided with a trend for established comedians, from Peter Serafinowicz to Alexei Sayle, to appear on children's TV in sketch shows like Horrible Histories, Sorry I've Got No Head and Fit.
Other performers doubling up with children's and adult shows at this year's Fringe include Howard Read, who was among the first stand-ups to tailor a show for youngsters.
Irish comic David Burke is doing a grown-up show on top of another in which he becomes Captain Gingerbeard (he has a ginger beard).
Circus performer-turned-stand-up comedian Martin Mor is also performing two shows - one that can be rather filthy and confrontational, and the other titled Funny Stuff For Happy People.
"I've got a lot of material so none of the adult stuff would ever be [performed] anywhere near the children," he says.
"Sometimes adult stand-ups do their normal material [for children] but just don't swear. That's totally not suitable. It has to be written specifically for the kids.
"Also, a lot of people write down for children. I think children respond very well to not being treated stupidly.
"If treated right, they're a fantastic audience because their imaginations are so free."
Another comic doing two Fringe shifts this year is Bec Hill, who has her own stand-up slot as well as a double act with fellow comedian Tom Goodliffe, in which they play friends who muck about in a strange launderette.
Goodliffe says: "I think some of the best comedy for anyone at the moment is on children's TV - things like Horrible Histories and Little Howard's Big Question.
"That makes me laugh as an adult, so if you can do kids' stuff that makes adults laugh as well, I think you're onto a winner."
Bec & Tom's Awesome Laundry is the pair's first children's show and Hill says the experience has helped improve parts of her routine for adults.
"There were some bits that got a laugh, they were working," she says. "But the other day I just went over the top physically in the same way I do in the kids' show and the reaction was massive.
"It really helped. I thought, all right, it's OK to be silly in the adult shows as well."
Comedy Club 4 Kids is at Underbelly until 25 August. The Horne Section's Family Bash is at Underbelly until 10 August. Martin Mor's Funny Stuff For Happy People is at the City Café from 9-18 August. Bec & Tom's Awesome Laundry is at the Gilded Balloon until 13 August.