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?"