I found one small pink sock on the floor of the garage. This is unusual. K loses two socks at a time, not one. (It's M who discards one at a time.)
Sticking to family protocol--noticing rather than nagging--I asked K why her sock was in the garage.
She explained, "M-- put a centipede in it and I'm waiting for it to crawl out."
I then asked M, "Why did you put a centipede in K--'s sock?"
He answered, "To annoy her."
Duh. Had not occurred to me. But then, I have never been a brother.