Stale bread isn't dry: its starch has quietly turned back into crystals

Bread left on the counter turns hard and crumbly, and it looks like it dried out. It mostly didn't. In 1852 Jean-Baptiste Boussingault showed that bread sealed so it can't lose water still goes stale.

The real culprit is starch. Baking melts it into loose, water-filled tangles; over the following days the chains line up into crystals again and squeeze that water out. Cold speeds this up, so the fridge is the worst place for bread, and heating the crumb to about 60 °C melts the crystals and makes it soft again.