How can an unopened jar honey turn into a hard block of sugar without being exposed to the air?

How come if I put that jar in boiling water, it turns back into a liquid form?

I believe the moisture seeps through the glass itself. When you boil it, I believe that forces moisture back through the glass by expanding the glass with the heat and creating more space between the molecules.

I'm going to have to think about this one, but I'm not convinced by mommeytimestwo's explanation for three reasons:
1) Heating glass will not make it expand to the the point where it will be significantly more water-permeable.
2) I've done this with the top off the jar an it still worked. Considering the amount of steam that had to have made its way inside the jar this way, i'm doubtful that it has anything to do with water.
3) I'm pretty sure I read an article in Applied Spectroscopy that used infrared reflectance to analyze honey. Among other things, I'm pretty sure they showed that water content does not correlate with crystalization.

On the other hand, I don't have a better answer yet.

Honey will crystalize over time at room temperature.. the hard block that you mention.
It's essentially freezing. When you warm the jar up. It melts the crystals. It's a slow process but it'll again crystalize over time
a reversable process.
Paturized honey will not crystalize as readily I believe the heat cleaves some of the longer molecules and lowers the melting point.
Moisture does have an influence..higher moisture honey will not crystalize as quickly.
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Good stuff fresh from the hive.. We just harvested honey a couple of weeks ago. My sister-in-law's family hobby gets a little out of hand 2.5 drums of honey..