"That's easy" says the first worker "The telephone is the greatest innovation ever. It allowed people to instantly speak to one another across the street or around the world. Truly amazing!”
"No way" said the second worker "The airplane is clearly the greatest innovation. It shrunk the globe letting you get from one side of the world to the other in hours."
"You are both wrong" said the third worker "It is the Thermos!"
"A Thermos?" said worker 1 and 2 incredulously.
"Why a Thermos"
"Yes, the thermos" confirmed worker 3
"In the morning if you put something hot in the Thermos, when you open it in the afternoon it is still hot. But if you put a cold drink in it in the morning and open it in the afternoon it is still cold."
"SO?" cry the other two
"How does it know?"