"The time machine is ready. You may use it to cause the most good you can think of in the world."
"Oh, that's easy. I'll go back and-"
"Anything except that. Paradoxically, it's almost guaranteed to cause your own nonexistence."
"Oh, okay. Then I'll go back to 1990, break into Nintendo headquarters, and redesign the SNES control pad so the A button is on the bottom and B is on the left."
"Excellent idea. Please step inside."