If you don’t like potty humor, it’s probably best to quit reading now, because this is all about a sitcom scene that takes place in a bathroom.

Nothing particularly racy was happening on Stage 22 at Twentieth Century Fox in early December, when tonight’s episode of CBS’ How I Met Your Mother was filmed. But the scenes, featuring stars Alyson Hannigan (as Lily) and Jason Segel (as Marshall), are all about the need to, uh, go.

So why, if Lily has to go, is she crossing her legs and refusing to relieve her bladder?

Simple. She’s never done it in front of boyfriend Marshall and they can’t leave the bathroom or they’ll disturb buddy Ted (Josh Radnor), who’s making out with a new girlfriend on the living-room sofa.

After Marshall makes declarations of love more fitting in a film about a soldier going off to war, Lily finally succumbs.

In one take, Hannigan goes to sit on the toilet, waiting for the cameras to cease filming. "You gotta say cut!" she says, laughing. In another take, she turns to the toilet, pretending that she’ll do her business standing like a guy.

Good cheer is easy to come by this day on the set. It’s a Friday, and filming will end by 4:30 p.m., an early escape for cast and crew.

But what makes Mother unusual among TV comedies is that it’s successful at a time when most new sitcoms are not. It’s also a rare hybrid.

In the early days of television (think: I Love Lucy), sitcoms were filmed with three cameras in front of a studio audience.

Later the single-camera comedy (Malcolm in the Middle, Arrested Development, Scrubs) came along and did away with the studio audience and laugh track, allowing more opportunities to film on location.

Mother shoots on a soundstage that was once home to Dharma & Greg, but the bleachers for an audience have been removed. It’s still filmed like a traditional sitcom, which these days has four cameras rather than three.

Craig Thomas, who created Mother with Carter Bays, said that the filming format was born out of necessity. The pilot episode contained 60 scenes, far more than most 15-scene sitcoms.

"It became clear that if we were to load in an audience, they would have to sit there for, like, 72 hours straight. We’d have to hydrate them and feed them," Thomas said. "It was too ambitious, but that was the rhythm we wanted for the show."

Filming the pilot without a studio audience resulted in what the creators, studio and network felt were more true-to-life performances from the show’s cast. But without audience reaction to offer guidance, "You really have to learn to trust your instincts," said Pamela Fryman, the show’s director.

A longer version of finished episodes are screened on monitors for an audience that sits in bleachers on the nearby Stacked soundstage. Their reactions are recorded and used as the laugh track. Jokes or lines that fall flat are cut until the episode hits CBS’ target running time (about 21 minutes).

Multicamera sitcoms have a pre-tape day and a shoot day, but Mother films for three days each week, making it both less conventional and more expensive. The network and studio were willing to foot that bill, perhaps out of desperation. TV comedies have not fared well in recent years.

But last fall, NBC’s My Name Is Earl became an instant hit and Mother received positive reviews and decent, still-rising ratings. Given the current climate, that was a surprise, as was the show’s network home, CBS, not the first channel you look to for a sitcom with a cast of twentysomethings.

"We didn’t think it was right for CBS," Thomas acknowledged. But CBS was looking to draw more young viewers, and Mother was a show that executives thought might help accomplish that goal.

The series is narrated by an older Ted in the year 2030 (voice of Bob Saget), who’s telling his children the story of how he met their mother. But it’s taking him a long time to get there: In the pilot, viewers were surprised at the end of the episode to discover that Robin (Cobie Smulders), Ted’s love interest, is not the mother of his children.

Will producers save that revelation for when ratings begin to drop?

"You mean sort of a ’break glass in case of emergency?’ We don’t think of it that way at all," Thomas said.

"We do have a plan in mind of how to do it, but we’re remaining mum about it."