Who's trunks are these?

We had been swimming over at my Mother-in-Law's pool, my wife her sisters and all our kids were there to visit and swim too. So, of course, suits were expected. A couple days later my wife was getting ready to wash the swim towels. " Who's trunks are theses?" (Thinking maybe one of our sons had left them behind .) "They're mine" I said reassuringly. She looked at my kinda funny and replied, "Oh, I guess I forgot you even owed a pair."