Calvin and Hobbes by Bill Watterson for June 27, 1990
Hobbes: I guess if we get to have snacks, it would be ok to time travel. If they're good snacks, I mean. Calvin: Great! Put on your vortex goggles. The dial is set for 140 million years ago, so off we go-o-o! Hobbes: I have a question. Why don't we get younger as we go back in time, and disappear as we pass the day we were born? Calvin: I'd explain it, but there's a lot of math. Hobbes: I thought you got a "D" in math.