How I Met Your Mother Season 9 Episode 12 has been all about stretching the sitcom limits: All 24 episodes take place over the course of one wedding weekend, and in another narrative twist, Monday night’s half-hour (8 p.m. ET on CBS) will play out entirely in rhyme.

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Last week, EW was on the set of the show’s 200th episode — premiering at the end of January — and we had to ask a few questions about the clever concept. So far, we know that the episode (titled “Bedtime Stories”) has Marshall (Jason Segel) trying to get baby Marvin to sleep with some very personal nursery rhymes.

“The structure of the flashbacks are that Marshall is recounting crazy things that happened to each one of us,” Neil Patrick Harris told EW on set, adding that two of the three acts involve Barney and his fiancée Robin (Cobie Smulders). “There’s a great adventure that Robin tells about this crazy night where she had superhuman powers, and then Barney recounts this dream tale of how he is the ultimate player in New York City.”

The third act revolves around Josh Radnor’s Ted, and the outside-the-box storytelling technique thrilled the actor. “I just thought that was such an imaginative episode,” he said. “That doesn’t take place [over the wedding weekend]. There’s a bunch of flashbacks. … It’s a great episode. Super weird. Never been done before in television.”

Alyson Hannigan had to resist slipping into mommy mode while reciting Lily’s rhyming lines. “It was hard not to do it like Dr. Seuss, to make it conversational,” she said. “I read a lot of children’s books. … It was really hard not to do it like I was reading to my kids.”

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But HIMYM creators Carter Bays and Craig Thomas were surprised at how simple it was to channel their Seussian voices. “It was so much easier than it would seem, to the point that what was difficult was writing that next episode and not being able to rhyme,” Bays said, while Thomas joked: “That’s why the rest of the season after this episode all rhymes. We couldn’t go back. Once that switch is flipped, you can’t go backwards.”

The pair really just wanted to keep challenging themselves in this final season to do any and every weird idea that came to them. “A good season 9 episode has to be an episode we’ve never done before, that we can only do this season, and that’s one of them,” Thomas said. “I think a lot of these season 9 episodes pass that test, if not all of them. … It was hard to switch out of rhyme mode. We rhymed for days afterwards.”

“You’re getting a coffee in the kitchen and you’re having a conversation with someone and feel weird that you’re not rhyming,” Bays laughed.

How I Met Your Mother airs Mondays at 8 p.m. ET on CBS. Watch a promo for the episode below: