The staling of bread and other starch-based foods is due to a process called retrogradation. In the bread-making process, water hydrates the starches in the flour and then, as the loaf bakes, the starches gelatinize and soften. Over time, the starches in that baked loaf crystallize and incorporate water into the crystalline structure, leading to an apparently dry, stale loaf. The good news is that the water doesn’t travel very far: Most of it remains trapped within the starch crystals. This proximity makes the retrogradation process reversible, at least for a little while.