A Thermos is made by encapsulating an airless vacuum between two steel or glass walls. This vacuum inhibits heat transfer between a liquid kept within an insulated glass container in the vacuum area and the outer case that remains cool to the touch regardless of the temperature of the liquid inside. The concept of the thermos flask was invented in 1892, and the Thermos company began commercial production in 1907.
Because a vacuum is one of the best insulators in science, it is used as the basis for making a Thermos flask. To make a Thermos, a container is produced out of steel or glass with dual walls. Between the two walls, an airless vacuum space is created. The existence of this vacuum prevents any heat transference from the liquid poured into the container, whether the liquid is hot or cold. The glass or steel container is then encapsulated in an outer shell that contains some additional insulation to pad and protect the inner container. The outer shell remains cool to the touch even if the liquid inside is near-freezing or boiling because the vacuum space surrounding it does not permit heat transference. When a liquid inside a Thermos loses some of its heat or cold, it is typically due to leakage of heat through the Thermos cap.