You know what to do now. That’s right — fill your glass with water at room temperature, tape it into the box, place your thermometer into the water, and put the whole works into your refrigerator. Read the thermometer every three minutes or so, and note how quickly the temperature drops.

OK, let’s add “storm windows.” Tape (again with cellophane tape) four more pieces of plastic wrap over the box openings. . . this time from the outside of the box. It is important that both inside and outside “windows” be taut so that there is a uniform, thin layer of air trapped between them.

Now, repeat the experiment. You will find that the water temperature drops slightly less quickly. The difference is small (although noticeable) because our imitation storm windows don’t work as well as the real thing.