[flashback to Barney and Marshall talking about "The Mermaid Theory" at work:]
Barney: Once mermaid-ified, there's only one way a woman can go back to being unattractive again, but it's pretty gruesome.
Marshall: Death?
Barney: Worse. Pregnancy. If a baby's on board that train, it is headed straight back to Manatee City... where the grass ain't green and the girls ain't pretty.