lucky guess about pre-teen girls

I had a lucky guess with a couple of teens. Or pre-teens. They were both 11 they said. I talked to them for just a few minutes and found out commented that they were equally pretty and unique in appearance but they had the same style of clothing and I guessed they did some swapping. One piped up and said yeah, they did that when they were at school and she said, "Right now, she's wearing my shirt and I'm wearing her shirt." I said, "How did I know?" They were cute.