Hero of Alexandria probably lived and worked in the 1st century A.D. His native city, Alexandria - now in Egypt - was one of the centers of antique Greek culture and science. Although Hero is known mostly for his theoretical work, e.g. “Hero’s formula” for the area of a triangle, he was also the inventor of several devices. One of these was the aeolipile, also known as the Hero engine or Hero’s ball. This device is widely regarded as the predecessor of the steam engine, 1,700 years before James Watt’s invention. The aeolipile was a hollow sphere mounted on a closed copper vessel partly filled with water. The water was heated by fire under the cauldron, which produced steam. The vessel was connected to a hollow sphere by a pair of pipes that also served as the pivots for the sphere and conveyed the steam. There were two curved nozzles on the sphere, where the steam could escape. The outflowing steam produced a torque, which rotated the sphere around its axis, that is, the device converted heat energy into mechanical work. Although Hero knew the working principle of pistons, he did not make the connection between the two devices. The aeolipile was never put into practical use, even Hero himself regarded it as a curious toy. Even so, the device is the first jet engine in the history of technology.