When I first heard it, it was billed as a riddle, not a joke. It only really makes sense in any way as a riddle / brain-teaser if it successfully plays on your

When I first heard it, it was billed as a riddle, not a joke. It only really makes sense in any way as a riddle / brain-teaser if it successfully plays on your biases of the operating room doctor being incapable of being his mother (or other father, etc). In a 1950s, old-timey-wimey, nuclear family mindset, the father is the breadwinner and the mother is the homemaker, and there is no other reasonable possibility. Anyone that was taught to think that way will hear the father dying in the car accident and then be confused when someone else can also refer to them as their son. This came up a few years ago for the first time for me, and I tried it on my (elementary school) sons and they immediately said "uhh, its his mom, I guess?" Anyway, I hope that clarifies things for you.