Silvered glass can keep tea hot for twelve hours

To keep tea at a steaming ninety-five degrees for half a day, a thermos relies on the physics of a perfect void. The flask consists of two glass walls with the air sucked out from between them, creating a vacuum that prevents heat from escaping through touch. To stop the heat from escaping as light, the glass is coated in a thin layer of silver. This metallic mirror reflects ninety-seven percent of infrared energy back into the liquid, effectively trapping the warmth in a loop.