The Brennan torpedo — a 19th-century version of the weapon — was launched by pulling a cable out the back. As counterintuitive as it sounds, pulling this cable backward propelled the torpedo forward. To show how this is possible (while side-stepping the messy specifics of how the turning propeller thrusts the vehicle forward), Steve Mould walks through a lever-and-pulley-based equivalent to the torpedo. He demonstrates that the key to the torpedo’s forward motion is a clever use of mechanical advantage. (Video and image credit: S. Mould)