True story. I staggered into a bar one Sunday at lunch to meet a friend. I was staggering not because I was already drunk, but because I was sore from a 22-mile run I'd done the day before through mountainous terrain.
"You look great," he observed as I stiffly winced my way up onto the bar stool. "Want to go to the museum later?"
"Can't. I'm going for another long run this afternoon."
"Why?" he asked, not unreasonably.
I responded much less jokingly than this statement should be delivered: "To punish my legs for hurting me."
He thought about this and drank some more beer.
Finally he offered, "You should study the mind-body problem."
"Because you have it."