Barney’s idea is a questionable “theory” and, like many Barney scenes, it hasn’t aged very well. But in the process of filming it, Cobie Smulders had to put on a ridiculous manatee outfit that made movement nearly impossible. In the blooper reel, at about the 1:25 minute mark, she’s sitting at the table with her co-star Jason Segel when she asks him for help picking up her cup of what is presumably water.
She’s trying to take a sip of the drink while maneuvering her giant manatee flippers, with Segel holding it for her, but instead of drinking it like normal, she just goes in with her tongue. It’s a hilariously awkward moment that evokes imagery of a cat lapping at a bowl, but then she just looks straight at the camera while sticking her tongue out — and that’s when the cast and crew start to lose it. Their laughter can be heard in the background while Smulders makes unwavering eye contact with the camera, licking water from the cup, making for a great moment in the reel.
In the actual scene, Robin merely picks up the glass by herself, but doesn’t attempt to drink from it. Of course, in “reality,” Robin is not in a manatee suit — that part is in Marshall’s imagination — so acting encumbered by it wouldn’t make sense in the show. Still, we can enjoy her out-of-character moment of silliness on the thoroughly amusing blooper reel.