Greatest Invention Ever

So this TV station reporter was interviewing a 103 year old guy known as the "oldest person in the County". Among other things, she asked him what he thought was the "Greatest Invention" of his lifetime. He says "Oh, that's easy. It's the Thermos bottle." The startled reporter says "Really? Why?" The old guy says "It keeps things hot, don't it?" She says, yeah, of course. He says "It keeps things cold, don't it?" Yeah, she says, they do. Right, the old guy says. "How does it know?"