What causes bread to become stale after a few days?

Bread becomes stale primarily through a process called starch retrogradation. When bread is baked, starch granules absorb water and swell in a process called gelatinization, giving fresh bread its soft texture. As the bread cools and ages, those starch molecules gradually recrystallize and expel moisture, causing the crumb to become firm and crumbly. This process happens fastest at refrigerator temperatures, which is why storing bread in the fridge actually accelerates staleness despite keeping it mold-free. Moisture also migrates from the crumb to the crust over time, making the crust soft and chewy while the interior dries out. Warming stale bread briefly in an oven can temporarily reverse retrogradation and restore some softness.