Very observant! I'm not sure I would ever have figured out the sorting criterion, there, unless he said something about it.

DS11, at 2.5, was obsessed with license plates. He knew every one in our apartment complex, and he knew people by their license plate number. He couldn't tell you what kind of car anything was, but he knew whose it was by the plate. When DD was born, he was 3.5, and he went off with friends for the day; he came to see me in the hospital, gushing about the "111xxx" motorcycle he saw, and the "222zzz" truck he got to ride in.