These days kids have Styrofoam to keep soup hot, but back in my day, we all had thermoses. One of their drawbacks was that since their insides were made of glass they were pretty easily broken when dropped. The popping sound they made when they broke was cool to kids, but then we weren't the ones who had to buy a new one.
A thermos keeps things hot not because it generates heat like a radiator, but because it doesn't allow heat to escape. What's the best way to prevent heat from escaping? Insulation. You can wrap the hot thing up in a material that doesn't conduct heat well. Then the soup won't be able to cool off easily, because the heat can't escape into its surroundings.
What's a good insulator? How about air? Air is, in fact, a pretty good insulator, which is why styrofoam keeps things hot. It's full of air.
However, even better than that would be a vacuum. Because there's nothing in it to heat up, heat can't conduct through it. A vacuum is the best insulator of all, and that's the principle behind a thermos.
A thermos is a flask with a double-walled container inside it. Air is removed from the space in-between the walls. Now heat can't escape from the soup, since it's surrounded by a vacuum. To further help hold in heat the inside walls are silvered to reflect heat rays which try to escape.
The thermos was invented in 1892, and is still in use.