The experimentalist comes running excitedly into the theorist's office, waving a graph taken off his latest experiment.
"Hmmm," says the theorist, "That's exactly where you'd expect to see that peak."
Here's the reason: A long logical explanation follows, but in the middle of it, the experimentalist says "Wait a minute", studies the chart for a second, and says, "Oops, this is upside down." He fixes it. "Hmmm," says the theorist, "you'd expect to see a dip in exactly that position. Here's the reason...".