Rhett asks a straightforward question over at Dot Physics, in A blanket and cold stuff

Suppose you put take two identical cans of soda out of the fridge and place them on the floor in the middle of a room. One can you leave alone and one can you cover with a wool blanket. After an hour, you come back and check on the two cans of soda. Which will be warmer?

The reason I think it’s a great question is that it plays on a common misconception about thermodynamics, and it reminds me of a joke, and a related story.

The joke is about a person declaring that a thermos (which we often call a Dewar) is the smartest thing in the world. When it’s pointed out that all it does is keep hot things hot and cold things cold, the response is, “How does it know?”

Well, some years ago we were giving a tour of our lab to an Admiral (or Sneetch. I have taken to calling military lab visitors Sneetches, a Dr. Seuss creature. The high-ranking ones have stars upon thars) We mentioned the vacuum system, and the Admiral mentioned the lines from the joke — it keeps hot things hot and cold things cold, how does it know? My colleague was so focused in on explaining things, he didn’t recognize the joke, and started explaining the physics of heat transfer: a vacuum is a really good barrier to nonradiative heat transfer. I mentioned that it was a joke before he went in for a second round of explanation.

But the misconception — that blankets heat things up, rather than act as a barrier to heat transfer, is what is pointed out in this example.