Based on a steam-driven toy developed by Hero of Alexandria around 150 BC. Perfect for a fascinating demonstration of jet propulsion. Our version consists of a spherical glass flask (10cm in diameter) with a rubber stopper. Two small tubes extending from the flask act as jets. A collar around the neck lets you hang it above a burner. When water is boiled in the flask, the steam escapes through the jets, causing the flask to spin. Eventually, the cord is twisted far enough to overcome the power of the jets, and the Hero's Engine turns the other way.