How does it know?

An engineer, a physicist, a mathematician, and a mystic were asked to name
the greatest invention of all time.
The engineer chose fire, which gave humanity power over matter.
The physicist chose the wheel, which gave humanity the power over space.
The mathematician chose the alphabet, which gave humanity power over
symbols.
The mystic chose the thermos bottle.
"Why a thermos bottle?" the others asked.
"Because the thermos keeps hot liquids hot in winter and cold liquids cold
in summer."
"Yes - so what?"
"Think about it." said the mystic reverently. "That little bottle - how
does it know?"