Quote from Phoebe in The One in Vegas (Part 1)
Phoebe: So, now, why can't we tell Chandler? Monica: Because it'll totally freak him out and tomorrow's our anniversary, I just don't want anything to spoil that. Phoebe: I can't believe you guys have lasted a whole year. Monica: I know. Phoebe: Wow, I owe Rachel 20 bucks. Monica: What? Phoebe: On a totally different bet.