I would venture a guess that many people in the United States have more than one thermos in a cabinet that boasts about its ability to keep hot things hot and cold things cold for a decent amount of time.

Have you ever wondered how the thing knows whether to stay hot or cold, though?

First of all, your thermos is not sentient and so doesn’t actually know anything at all. Instead of magical enchantments, the thermos uses a little thing called thermodynamics to get the job done.

The technology entered its early development phase when 19th century Scottish chemist and physicist James Dewar invented a machine capable of yielding large quantities of liquid oxygen. Since it boils around -297°F, though, they had no way to keep it cold long enough to study it.

The second law of thermodynamics dictates that heat always flows from hotter to colder areas. Heat is lost in three ways: conduction, convection, and radiation.

Conduction happens through direct contact, where energy transfers through collisions between molecules and atoms. Convection takes place when a hotter fluid rises to the top of a system while the cooler fluid sinks, creating convection currents. Radiation takes place through electromagnetic waves that form as atoms move or vibrate.

Hot coffee in a regular cup loses heat through conduction. The heat transfers into the sides of the cup and the air above, losing a little of its energy by the moment. Convection plays a smaller role, as the hotter coffee swirls up to be cooled by the open air.

In 1892, Dewar came up with the idea of placing one flask within another, separating the two by a near vacuum. That vacuum layer prevents both conduction and convection – he even applied a reflective material inside the interior flask to slow radiation as well.

The thermos doesn’t need to know if your food is hot or cold, then, because the system slows it from being heated or cooled, and simply keeps them the temperature it was when they went in.

Sadly, Dewar did not realize the gold mine he had discovered and never filed for a patent.

It’s called a thermos after the company that did obtain a patent some time later.

If you think that’s impressive, check out this story about a “goldmine” of lithium that was found in the U.S. that could completely change the EV battery game.