Science & TechHow it works
A thermos works because there's nothing inside it.
Heat travels three ways: conduction, convection and radiation. A thermos has two walls with a vacuum between them. With no air, there is neither conduction nor convection. Against radiation, the inner surfaces are mirrored. What remains is only the thin neck where the walls connect. James Dewar invented the vessel in 1892 for liquefied gases.
Useful: Pre-warming it with hot water helps, because otherwise the cold inner wall eats the first few degrees of your coffee.
Source: en.wikipedia.org