Stale bread isn't dry (even if it seems so)

We take it for granted that bread hardens because it dries out, because it loses water. But that isn't the main thing: bread can go hard even wrapped up with all its moisture still inside.

What happens is subtler. When bread comes out of the oven, its starch is disordered and fluffy. Over the hours, those molecules slowly rearrange themselves into small rigid structures, a process called retrogradation. The bread isn't drying: it's recrystallising from the inside.

Here comes the useful surprise. That rearranging goes faster in the cold, so the fridge is the worst place for bread: it hardens it sooner. Heat, on the other hand, undoes the process: that's why a day-old roll turns tender again for a few minutes if you warm it up. And the freezer stops it dead. Old bread hasn't lost water. It has only tidied up its molecules.