If you were asked to guess when the steam engine was first invented, you'd probably guess around the 18th century, powering as it did the Industrial Revolution. 

And you'd be right to – but well over a thousand years before that, shortly after 0 CE, Hero of Alexandria described a sort of steam engine, used for an unknown purpose. The device is a simple steam turbine, which turns when the water container is heated.

"Then it will follow that, the cauldron being warmed, the steam (atmis) falling into the sphere [...] will fall out through the bent-back (little tubes) to the lid and will turn the sphere, just as in the case of the dancing figures," Hero, aka Heron, wrote. 

The devices were replicated for a while in the 16th century as Heron's work was popularized, and then again for a fun and simple science lesson centuries after that.

While a cool device to show all your Ancient Greek buddies, it had little in the way of actual use.

"Heron's device is inherently the wrong design to produce much in the way of useable power," Paul Keyser wrote in A New Look at Heron's 'Steam Engine', which explored how the machine worked.

"The rotating sleeve joint [...] must either have excess friction or excess leakage, in either case reducing the efficiency of the device. The sphere would have insufficient inertia to give a smooth output for varying load (i.e., it is a poor flywheel), and it would spin too fast (over 1,000 RPM) to allow simple reduction gearing to transmit the power."

Keyser calls the engine – which is far from the true steam engines created centuries later – instead the first demonstration of a rocket and reactive force.

So, what was its purpose? Frustratingly, Heron was quiet on that front, leaving it to speculation. It's thought that the device was probably used as a novelty item to, well, amuse and astound Ancient Greek buddies. Beyond that, the aeolipile could have been used to create miracles in temples.

“A boiler hidden in the hollow figure of an idol would produce steam through a pipe ending in one or two branches through the nose/ or mouth," Harry Kisikopoulus wrote in Innovation and Technological Diffusion: An Economic History of Early Steam Power, as per Popular Mechanics. “The escaping steam giving the impression of a breathing figure, inspiring awe.”