Bread gets firmer and less pleasant after sitting for a while.
It is tempting to say the bread simply dried out.
Moisture matters, but that is not the whole explanation.
A major part of staling happens inside the starch.
Baking changes the starch
When bread bakes, starch granules absorb water and change structure.
That helps create the soft crumb of fresh bread.
Once the loaf cools, those starch molecules begin reorganizing.
This gradual process is called starch retrogradation.
The structure becomes firmer
As starch chains move into more ordered arrangements, the crumb becomes less soft.
That can happen even when bread is stored in a sealed bag.
So staling is not just water escaping into the room.
Moisture also moves around
Water shifts between different parts of the bread.
The crumb can lose some moisture while the crust absorbs moisture and becomes less crisp.
That is why stale bread may have a firm interior and a disappointingly soft crust at the same time.
Why does warming stale bread help?
Heat can temporarily disrupt some of the ordered starch structures.
That makes the bread feel softer again.
Toast, a warm roll, or reheated bread may seem surprisingly fresh for a short time.
The improvement is temporary because the starch reorganizes again as the bread cools.
Is stale bread the same as moldy bread?
No.
Staling is mainly a physical and chemical change.
Mold is biological growth.
Bread can be stale without being moldy.
It can also grow mold before it becomes terribly stale.
Those are different problems.
How is this different from why bread rises?
The earlier process behind bread rising involves gas production and the dough’s structure before and during baking.
Staling happens afterward, as the baked starch and moisture continue changing.
The loaf is still doing chemistry long after it leaves the oven.
So stale bread is not merely sitting there getting dry.
Its starch is reorganizing itself into a firmer structure.
And that’s the short explanation.
Related explanations: Why does bread rise?, Why does toast turn brown?, and Why do potato chips go stale?