The Calvin and Hobbes Wiki
Reversed Gravity

Calvin figures out his gravity problem.

In a few strips, the gravitational pull of the Earth inexplicably becomes reversed on Calvin.

In a Sunday strip, Calvin was playing outside when he started being pulled up into the sky. He escaped by grabbing hold of a passenger airplane, and it took him very far before he regained normal gravity.

In a weekday strip, Calvin explained his failed homework assignment by claiming that his parents forgot to pay the gravity bills and he was so gravity-deprived, he was unable to work.

This plot point was reused in a story arc, where Calvin's gravity reversed as he was about to start doing his homework, and he was as such unable to complete his assignment.

Also, in a single weekday strip, gravity became completely absent in Calvin's house.