Quote from Phoebe in The One with Phoebe's Rats
Mike: Okay, fine.These rats are our responsibility. What happens when they mate and there are hundreds? Phoebe: Mate? They're all brothers and sisters. Mike: Yeah, not such a problem with rats. No, they're more of a "love the one you're with" kind of animal. Phoebe: No. Really? Oh, my- Get off your sister!