[flashback to Barney in Marshall's office explaining "The Mermaid Theory":]
Barney: [v.o.] It was 300 years ago. Sailors stuck at sea would get desperate for female companionship. It got so bad that, eventually, the manatees out in the water started to look like... beautiful women. Mermaids.
Barney: Let's go get some tail!
Barney: You see, every woman, no matter how initially repugnant, has a mermaid clock. The time it takes for you to realize you want to bone her. Sure, today you see Iris as a manatee. But she ain't gonna stay that way. Marshall, your secretary's mermaid clock starts right now. [clock ticking]
Marshall: [v.o.] And it took one year, three months, and 16 days, but eventually... D'oh!
Barney: Told you. The Mermaid Theory. It's a thing. You owe me 500 bucks.
Marshall: Did we bet on this?
Barney: Let's say yes.