One day, Jessica Brown and Arthur Brown, the parents of Jenny Brown (daughter) and Harry Brown (son), awoke to find one of their valuable vases broken on the ground. They immediately suspected that either their children, Jenny or Harry, or their Yorkshire Terrier, Hershey, had somehow accidentally broken it.
Both parents didn't want to punish anyone before they knew who had done it, so they investigated the "crime scene". They found a long piece of brown hair. That didn't help much, since oddly, all the Browns had brown hair, and so did Hershey. Unfortunately, that was all that they found.
After Arthur thought about it for a while, he knew who had broken the vase. Who broke it, and how did Arthur know?