Best Invention

Three guys sat around discussing the greatest technological advancement.
First guy says: "I think it wast the wheel, because man was able to use it to move stuff and be able to go farther"
Second guy says: "I think it was electricity, because with it, man created machines that used the wheel and and other stuff"
Third guy says: "I think it was the thermos."
The other two looked at him and said what do you mean?
"Well, he said, you put something hot in, it stays hot, right?"
The other two agreed.
"You put in something cold, it stays cold, right?"
The two agreed. But asked how that meant anything.
He says: "How does it know?"