The story of Bart and Lisa's rivalry is told when Homer finds an old film strip.
Family photos from six years earlier show that Lisa and Bart were bitter rivals, always fighting each other. However, when they were left in the charge of Grandma Flanders, whom they thought had died, they left the house in search of ice cream and Bart saved Lisa's life - leading to the on and off relationship that they still have.—don @ minifie-1