Calvin and Hobbes by Bill Watterson for October 30, 1993
Calvin: Dad, why do my eyes shut when I sneeze? Dad: If your lids weren't closed, the force of the explosion would blow your eyeballs out and stretch the optic nerve, so your eyes would flop around and you'd have to point them with your hands to see anything. Calvin: Gross. How come you know so much? Dad: It's all in the book you get when you become a father.