'Definitions' - Season 5, Episode 1
Ted begins a new career as a professor. Meanwhile, Lily forces Barney and Robin to have "the talk" and define their relationship.
Air Date: September 21st 2009.
Lily: We're at the point of physical violence! Now will you please have the talk?
Barney: Because of that? I'm always punching my guys. That's my thing. I'll punch a girl; hell, I'll punch a baby.
Robin: Look, Barney's awesome.
Barney: And Robin's more than awesome. She's awe-some-more. She's even awe-quite-a-lot.
Barney: How do you keep a girl from becoming your girlfriend? Simple! The rules for girls are the same as the rules for gremlins.
Barney: Gremlins. Rule number one: Never get them wet. In other words, donít let her take a shower at your place. Number two: Keep them away from sunlight, i.e. don't ever see them during the day. And rule number three: Never feed them after midnight. Meaning she doesn't sleep over, and you don't have breakfast with her. Ever.
Barney: Lily, why can't you just let us be happy?
Lily: You're not happy! You just think you're happy because you feel happy.
Barney: And that's not happy?
Lily: Of course not!