Staling

Staling, or "going stale", is a chemical and physical process in bread and similar foods that reduces their palatability - stale bread is dry and hard.
Mechanism and effects.
Staling is not simply a drying-out process due to evaporation. One important mechanism is the migration of moisture from the starch granules into the interstitial spaces, degelatinizing the starch. The starch amylose and amylopectin molecules realign themselves causing recrystalisation. This results in stale bread's leathery, hard texture.
Bread will stale even in a moist environment, and stales most rapidly at temperatures just above freezing. Bread stored in a refrigerator will have increased staling rates.
Countermeasures.
Anti-staling agents used in modern bread include wheat gluten, enzymes, and glycerolipids, mainly monoglycerides and diglycerides.
Culinary uses.
Many classic dishes rely upon otherwise unpalatable stale bread. Examples include bread sauce, bread dumplings, and flummadiddle, an early American savoury pudding. There are also many types of bread soups such as wodzionka (in Silesian cuisine) and ribollita (in Italian cuisine). A sweet dish is bread pudding. Cubes of stale bread can be dipped in cheese fondue, or seasoned and baked in the oven to become croutons, suitable for scattering in salads or on top of soups. Slices of stale bread soaked in an egg and milk mixture and then fried turn into French toast (known in French as "pain perdu" - lost bread). In Spanish and Portuguese cuisines "migas" is a breakfast dish using stale bread, and in Tunisian cuisine "leblebi" is a soup of chickpeas and stale bread.
Stale bread can be used to "stretch" meat in dishes such as haslet (a type of meatloaf in American cuisine) and garbure (a stew in French cuisine). It can be a subsidiary ingredient in dishes such as fattoush (a type of salad in Levantine cuisine). Stale bread can be used as a base for dips such as skordalia (in Greek cuisine), or substituted with another ingredient. 
In medieval cuisine, slices of stale bread, called trenchers, were used instead of plates.
Destaling.
Stale bread can be partially destaled by heating to 60 °C (140 °F) in a conventional oven or microwave oven.