Leave a baguette on the counter overnight and it turns hard, dull and crumbly. It looks like it dried out. Mostly, it didn't. The water is still in there. It has just been locked away by the starch, which spends the days after baking slowly turning back into crystals.
In 1852 the French chemist Jean-Baptiste Boussingault showed that bread goes stale even when it is sealed so tightly that it cannot lose any water. If staling were just drying out, sealed bread would stay soft forever. It doesn't.
There is a second clue in every kitchen: warm a stale roll in the oven and it turns soft again. A dry sponge doesn't get wetter when you heat it, so something inside the crumb itself must be changing, and changing back.
Flour is mostly starch, packed into tiny, tightly ordered granules. Starch comes in two kinds of molecule: amylose, long and mostly straight chains, and amylopectin, big bushy molecules with many short branches.
In the oven the granules soak up water from the dough, swell and lose their order. This is called gelatinisation. The chains end up tangled loosely around water, a bit like cooked spaghetti in its sauce. That soft, moist, disordered starch is what makes fresh crumb feel tender and springy.
As the loaf cools, the chains start to find each other again and line up side by side into tidy, crystal-like bundles. Food scientists call this retrogradation. The straight amylose chains do it fast, within hours of baking. That actually helps: it is part of what sets the crumb, which is why a loaf sliced straight from the oven feels gummy.
The bushy amylopectin is slower. Its short branches pair up and crystallise over the following days, and this is the main driver of staling. Every crystal is a place where water used to sit between chains. The water is squeezed out into the gluten and the air pockets, and some of it wanders out to the crust, which is why a crisp crust turns leathery while the inside turns firm and crumbly. The bread weighs almost the same. The water is just in the wrong places.
Most chemistry slows down in the cold. This process does the opposite. Between roughly 4 °C and 60 °C, the colder the bread, the faster the starch recrystallises, and staling is fastest just above freezing. Chains need to slow down enough to stick together, and at fridge temperature they stick eagerly while still being able to move.
So the fridge, which keeps milk and meat fresh, makes bread stale several times faster than the kitchen counter. Go colder still, into the freezer, and the water freezes solid. The chains can barely move at all and staling nearly stops, which is why frozen bread thaws out tasting close to fresh.
Starch crystals in bread melt when the crumb gets to around 60 °C. That is the trick behind reviving a loaf: a few minutes in a hot oven or a toaster and the chains let go of each other, soak the water back up, and the bread feels fresh again.
There is a catch. The fix is temporary, and every trip through the oven drives off some water for good. Reheated bread goes stale again quickly, so warm it just before you eat it. Once enough water has left, you get rusks and croutons instead of fresh bread.
Keep bread you'll eat in a day or two at room temperature, wrapped or in a bread box.
Slice it first, freeze it, and toast slices straight from frozen.
A few minutes in a hot oven melts the crystals. A splash of water on the crust helps. Eat it while it's warm.
Bakers fight retrogradation too. Fat, sugar, eggs and milk get in between the chains and slow the crystals down, which is why brioche stays soft longer than a lean baguette. Industrial sliced bread often contains enzymes that snip amylopectin's branches so they can't pair up as easily. Stale bread isn't dead bread. It's bread whose starch has quietly gone back to being tidy.