I am referring to the thermos, the metal bottle that keeps our drinks warm in the winter and our water cold in the summer. As a child, I believed that it worked with some exotic mechanism and some magical substance...

Imagine the SHOCK I had when I learned that it's actually just... empty?!

To understand how simple and effective the thermos mechanism is, we need to look at how heat is transferred in general.

This happens in three ways: Conduction, Convection, and Radiation. What are these again? Conduction refers to the transfer of heat when two objects come INTO CONTACT, like what happens when we put a pan on the hot stove eye.

Convection refers to the transfer through a fluid. When we leave our coffee in a cup, it heats up the air around it. The warm air moves, and new, colder air comes in to warm up in turn. That's why we blow on something to cool it down. We're pushing cold air onto the hot food.

We easily understand radiation. When we leave a glass of water in the sun, it will heat up. Literally, it's being hit by the sun's rays. It's the same reason why our side facing the fireplace gets burned and our back gets cold.

In the summer, when we want to keep our drink cold, we essentially need to prevent the heat from the environment from reaching our bottle. In the winter, we want the exact opposite to happen, for the heat not to escape from our drink to the environment. In both cases, the problem is the same, we need to stop the transfer of heat!

So how does the thermos achieve this? As I told you, a thermos consists of two walls, an outer and an inner one, with a vacuum in between. Not air, a vacuum! We have sucked out the air.

Since there is a vacuum, the outer wall comes into contact with the environment but not with our liquid. So, conduction (transfer through contact if you remember) is eliminated. After we have sucked out the air from the thermos' intermediate space, convection cannot occur either (like it does in our coffee cup).

And what about radiation? If you open a thermos, you will see that internally its walls are covered with a silver layer that literally reflects the radiation and does not let it pass through (or out). Like the firefighters' suits!

It still seems incredible to me how simple the mechanism is! I'll leave you with one last fact: can you guess why thermoses are mainly made of metal? It has to do with the vacuum. Think about what happens in an empty plastic bottle when you suck the air out of it. It crumples!