Robin, Paco, and Mary learn about the illusion of impact in stunts. In Vitry sur Seine (near Paris), Chantal visits a collision research area to observe the buffers on high-speed trains. David explains how to measure the coefficient of restitution of several balls. Maggie explains the forces at work in bouncing on a trampoline. RECYCLED FROM EARLIER IN SEASON 5: Linus Pauling remembers his first science lesson: how to sharpen a pencil.FILM INSERTS: The principle of seat belts; how a kangaroo is like a rubber ball.