Bread Goes Stale From Recrystallizing Starch

Photo by How can I recycle this, CC BY 2.0, via Wikimedia Commons

Bread goes stale through a process called starch retrogradation: the starch molecules that gelatinized (absorbed water and softened) during baking slowly realign into a more rigid crystalline structure as the loaf cools and sits, squeezing out moisture and firming the crumb — it's a structural change, not simple drying out. This is also why the freezer, not the fridge, is the better place to store bread you won't finish soon: retrogradation actually happens fastest at refrigerator temperatures, while freezing stops it almost entirely. A brief reheat in the oven can even partially reverse mild staling by remelting the crystals.