How does Thermos flask work?

The outer casing of a thermos flask is made up of plastic or metal. Plastic, being a poor conductor of heat, acts as an insulator preventing the heat loss through conduction. Inside it, is a double-walled container made of glass or stainless steel.

Both the walls are silvered so that they are shiny. The space between the two walls is a vacuum; this prevents prevents heat loss through convection since there are no air particles to carry out the transfer of heat. the shiny surface of the walls also prevents heat loss through radiation.

So, the three modes of heat transfer: conduction, convection, and radiation are prevented in a thermos flask.