A riddle for the ages
By 
When I was ten years old in 1970, my fifth grade teacher presented the following riddle to our class:
A man is driving with his young son, when they get into a terrible accident. The man is killed, and the boy is seriously injured. The boy is rushed to the hospital for emergency surgery. The surgeon enters the operating room, takes a look at the patient, and says “I can’t operate on this boy — this is my son.”
I gather this was considered to be a genuine puzzler at the time.
I really have no larger point here, other than I just remembered this incident, and I wondered how difficult this riddle would be for ten year olds today.