A thermos keeps hot tea hot for hours. Riya says a thermos is only for hot things; putting ice or…

Why

Riya's seen a thermos only used for tea and chai, so 'keep-hot device' is a fair label from experience. But look at HOW it works, not what it's usually filled with. A thermos doesn't add heat; it slows heat from crossing its walls in EITHER direction. For hot tea, it stops heat escaping. For ice, the enemy is outside heat getting IN and melting it — and the same walls block that too. So it keeps cold things cold just as well. Its job is 'stop heat moving', which cuts both ways. Test: carry ice water in a thermos on a hot day — it stays icy for hours, proof it isn't a 'hot-only' tool. Riya's not right: a thermos slows heat both leaving and entering, so it guards cold as faithfully as it guards hot.