Robin as a manatee (becoming a mermaid) to Marshall.
- For the episode revolving around this theory, see The Mermaid Theory.
The Mermaid Theory is the theory that a man will eventually want to sleep with any woman after a period of time, no matter how initially repugnant. The theory is so called because sailors (according to Barney) would spend so long at sea that they would eventually come to view manatees as beautiful mermaids.
A mermaid will become unattractive (turn into a big ugly manatee) again by becoming pregnant. However, she will become a mermaid once again by breastfeeding.