[flashback:]
Robin: I'm sorry, I missed the first half of this. Why is picking up a drunk chick sitting alone and rubbing a tan line on her finger where a wedding ring used to be a challenge?
Barney: Because I said, "Challenge accepted." Pfft.
Lily: See, that's your problem, Barney. You only accept challenges you came up with. Just once, I'd like to see you accept someone else's challenge.
Barney: Challenge accep... Wait, what's the challenge?
Robin: Interesting. An opportunity rife with possibilities. We need to find something with just the right amount of poetic...
Lily: Get a girl's number while talking like a dolphin.
Robin: That works.
Barney: [imitates dolphin: "Challenge Accepted"]