Robin: Pregnancy?
Marshall: Mm-hmm. Once a mermaid gets pregnant, she becomes a manatee again. Never thought I'd say that sentence.
Future Ted: [v.o.] Wait! That's it! That's what Barney and Lily were fighting about! The Mermaid Theory! But their argument didn't happen at the same time as all this other stuff. I'm literally in the wrong year! It happened much, much later when Lily was... pregnant.
[flashback to Barney and pregnant Lily in the apartment:]
Barney: Well, Lily, I guess it's just you and me. You wanna hang?
Lily: Nah.
Barney: [gasps]
Lily: Whenever we're alone, you spend the whole time undressing me with your eyes. You even take off my shoes.
Barney: High heels chafe my shoulders. But you don't have to worry about that anymore. Ever since you got pregnant, you're just a big fat manatee.
Lily: [gasps] Well, you're a big... stupid octopus face!
Future Ted: [v.o.] That was it! I remember now!
[flashback to Barney and pregnant Lily in the street:]
Lily: Wow. Thanks, Barney. You saved me. You saved us.
[flashback to Barney and pregnant Lily at MacLaren's:]
Lily: If there's one thing you never do, it's call a woman "fat" right to her face!
Barney: I'm sorry, Lily. Hey, you want to see a magic trick?
[Barney puts the beer bottle on Lily's stomach]
Lily: Oh... You're a jerk.
Barney: Hey. Hey, wait. Come on. I was kid... Wait, wait, wait! There's another addendum to The Mermaid Theory. A pregnant woman who's become a manatee can become a mermaid again through one simple act.
Lily: What's that?
Barney: Breast-feeding. Hot.
Lily: Really?
Barney: Really. When those things swell up to three times their normal size, so do I.
Lily: That's so sweet!
Future Ted: [v.o.] See kids? Told you I'd get it!