The Mystery of the Thermos

A newspaper reporter went to interview an old man who was the last person in the county to have lived through the Civil War. Thinking he had a story, the reporter started asking some questions. "Sir, you have lived through the civil war and two world wars. You have seen the invention of the automobile and the airplane. In all of your days, what would you say is the single most interesting thing you have seen in all of your days?"

The man replied, "The Thermos."

Puzzled, the reporter asked, "Why?"

"Well," the old man responded, "when you put something hot in it, it keeps it hot. When you put something cold in it, it keeps it cold... HOW DOES IT KNOW?"