An over-glorified thermos. A dewar is a bottle used to hold liquid nitrogen or liquid helium. It's generally made of silvered glass with a vacuum insulating the interior surface from the exterior.

When you fill a dewar with liquid nitrogen, the nitrogen boils and steams until the dewar cools down. Then, you can hold your breath and look down to see the liquid nitrogen at the bottom. But whenever there is a mist blocking the view, I always make the mistake of trying to blow it away. Bad plan! It's not smoke, it's steam.

Update: Thanks, hands, but I know it'a not really steam. See Webster 1913's second definition: "The mist formed by condensed vapor; visible vapor; -- so called in popular usage." I think it sounds best this way.