“Did I pass, doc?”
“You’ll be pleased to know, you’re in perfect health.”
“Sweet! I’ma be an astronaut!”
“Well . . . no.”
“!!! . . . Why not? You said I’m in perfect health!”
“You have type A-positive blood. Astronauts are required to have type O-positive.”
“That seems arbitrary.”
“No; it’s necessary. If someone got hurt, you couldn’t be a blood donor. The body of any O-type person would reject it. O-positive is the largest blood group type, so we picked that. Sorry.”