Calvin and Hobbes by Bill Watterson for March 19, 1991
Transcript:
Mom: I heard Calvin splashing in the tub, but there's no water on the floor. His towel is hung to dry! The toothpaste cap is on! There's no mess anywhere! And you're already in bed?? Calvin: Would you check over my homework tonight, so I can correct any mistakes in the morning before school? Thanks, Mom.
Obviously the Calvin from a parallel universe has accidently swapped places with him.