Bread left out for a day turns firm and crumbly, and most people describe this as drying out. The main process is different, and understanding it explains several odd storage results.
Staling is recrystallisation, not moisture loss
During baking, starch granules absorb water and swell, losing their ordered structure. That disordered state is what gives fresh bread its soft, yielding crumb.
As the loaf cools and sits, the starch molecules gradually rearrange into an ordered form again, trapping water in a rigid structure that feels dry to the bite.
The water is still present. A stale loaf and a fresh one of the same age can hold similar moisture, yet only one of them feels moist, because texture depends on how the water is held.
Refrigeration makes it worse
Starch recrystallises fastest at temperatures a little above freezing, which is precisely the range inside a domestic refrigerator.
Bread stored there stales several times faster than bread left at room temperature, which is the opposite of what people expect from a device that preserves most other foods.
Freezing avoids the problem because the process effectively stops well below zero, so a frozen loaf holds its texture and resumes staling only after it thaws.
Warming reverses part of the change
Heating stale bread above a threshold temperature breaks the recrystallised structure apart again, which is why a briefly warmed slice regains much of its original softness.
The recovery is temporary. Once the bread cools, recrystallisation resumes from where it left off, and it proceeds faster the second time.
Repeated reheating also drives off genuine moisture, so each cycle leaves the loaf drier than the last and the improvement becomes progressively smaller.
The crust follows the opposite path
A fresh crust is dry and crisp because baking drove moisture out of the outer layer while the interior stayed wet.
Over the following hours, water migrates outward from the crumb into the crust, softening it while the interior firms. The loaf becomes uniform in the least appealing direction.
Wrapping a loaf accelerates crust softening by trapping that migrating moisture, while leaving it exposed keeps the crust crisp at the cost of genuine drying at the cut face.
Formulation changes the timescale
Fat, sugar and certain enzymes interfere with starch recrystallisation, which is why enriched breads and commercial loaves stay soft for far longer than a lean bread does.
Sourdough fermentation has a similar effect through acidity and the breakdown of starch during a long proof, giving a slower staling curve without additives.
A plain flour-water-salt loaf sits at the fast end of the range, which is the trade-off accepted for its flavour and crust rather than an oversight in the recipe.