Everyone knows...

"Everyone knows" to keep your wagon moving you have to keep pulling. The notion that something in motion will keep moving until a force acts to stop it was not obvious.

It is even less obvious that heat is a form of momentum, of motion. When we compress a gas, it gets smaller and hotter. Just as momentum is passed from the left to the right side of "Newton's cradle", momentum is passed in and out of a vapor if it is compressed and decompressed. Momentum is transferred between vapor and machine.

Sadi Carnot was the first to identify this process is "perfect" or "reversible". No matter how many times a vapor is expanded or compressed, the same amount of heat or work is exchanged each time. (It is important that the vapor is insulated. Heat loss from heat diffusing into its container is the primary problem with making Heat Engines efficient.) This observation was one of the key discoveries that led to the Three Laws of Thermodynamics.