They were fighting crime or something. After they finished, they started heading home, and then they saw you, a.... cat?

Donnie: Why is there a cat on the side of the road?

Leo: it's New York, I'm not surprised

Raph: Awee, maybe we should take it in

Donnie: No offense, but none of us, except me, of course, are suitable for taking care of a cat

Mikey: Well, to prove ourselves, how about we take it in, and then we each have a day taking care of it? To prove that we can care for a kitten?