I first heard this riddle on All In The Family in the 1970s. It of course completely stumped me and my whole family, progressive though we were.

I ask it of people periodically to see how humans are doing with respect to gender. I have been gratified that in the last fifteen years, an awful lot of people who had never heard it before not only jumped right to the solution, but were baffled that anyone would even consider this puzzling.