What I want to know is just *how* the thermos knows that the stuff in it is supposed to be hot or cold?
Anyone???
It doesn't. How could it? Why should it?
Why overcomplicate things?
The vacuum bottle is merely an insulator. It inhibits energy passing through it.
Hot coffee? Lots of energy inside, wants to get out but can't get out because vacuum bottle. Therefore coffee stays warm.
Ice tea? Very little energy inside, wants to suck in energy from outside but can't because vacuum bottle. Therefore tea stays cold.
As I’ve said before, it’s the same way the deep water knows to stay out of the shallow end of the pool.