dean wrote:
Are they air tight? I mean, even after several years they still don't
get misted up inside, but I can't imagine how (with all the cold and
warm contractions and expansion) they ever manage not to leak somehow?
Am I missing some kind of magic here?

No, you're not missing anything except more exposure (time) with the
windows. They don't last forever. If the seal fails, you'll get
fogging on the inside of the glass. The sealed units are usually
warrantied for five or ten years, although it is not unusual for sealed
units to last for twice that or more.

R