Very sad story

Santa and Banta are flatmates who live on the tenth story. They reach their apartment building and park the car. When they enter the apartment building they find out that the elevator is not working so they decide to take the stairs. Santa tells banta to tell a story to each other to pass the time while they are climbing the stairs. Banta tells him a story and asks him to take his turn and tell a story after reaching the 3rd floor. Santa says that his story is very sad and he will tell it later. So Banta tells another story and they reach the 7th floor and he asks Santa to tell a story, but Santa says that his story is very sad and asks Banta to tell another story to pass time. Banta starts telling another story and they reach the 10th floor at their apartment. Banta insists that Santa tell the story now that they have reached home. Santa says, "This is a very sad story, we forgot our house keys in the car."