How a Thermos Keeps Drinks Hot or Cold

A thermos can keep coffee hot for most of a workday or ice water cold for an entire afternoon, and it manages both jobs using the exact same design, since heat naturally moves from hot areas to cold ones no matter which direction you're trying to prevent it from moving. The core of a thermos is a container built with two walls instead of one, with a thin gap of empty space, a vacuum, sealed between them. Heat can transfer between objects in three ways: conduction, which requires direct physical contact between materials; convection, which requires a moving fluid like air to carry heat along with it; and radiation, which travels as electromagnetic waves and doesn't require any physical medium at all. A vacuum contains essentially no matter, so it blocks both conduction and convection almost completely, since there's no material present in the gap to physically carry heat across it. Radiation is trickier, since it can cross empty space, which is exactly how sunlight reaches Earth through the vacuum between the sun and our planet. To block radiant heat as well, the inner wall of a thermos is typically coated with a thin, shiny, reflective layer, usually a metal like silver or aluminum. That reflective surface bounces radiant heat back toward its source rather than absorbing it, which is why a thermos looks mirror-like on the inside. Between the sealed vacuum gap and the reflective coating, almost no heat can escape a hot drink or enter a cold one, which is why the temperature inside changes only very slowly.