A knock at the door

There was a knock on the door. Jon ran back into his room. Although everyone thought that there was nothing wrong with him, Jon still felt self conscious in his new body. He head his Mom's voice. "Jon, Jeff's here to see you" Jeff was one of Jon's friends at school. Jon poked his head round the corner. "Hi there, I'll, er, be down in a minute" Just then, Jon had an idea. He looked at Jeff, and said "Swap". Suddenly, he had Jeff's body from the waist down, and Jeff had the model's body...