> a thermos is specifically good at insulating because not only does it have a vacuum gap, it's also got two layers of metal (inner and outer) to absorb and ref

> a thermos is specifically good at insulating because not only does it have a vacuum gap, it's also got two layers of metal (inner and outer) to absorb and reflect thermal radiation. Not necessarily. There are many modern thermos "cups" that are just a regular cup, except with two layers of glass and a vacuum. Even the top is open all the time. (e.g. https://www.ikea.com/us/en/p/passerad-double-wall-glass-80540293/ https://www.ikea.com/us/en/p/passerad-double-wall-glass-8054... ) It's still good enough to keep your coffee hot for an entire day.