Bread goes stale due to retrogradation, a process that happens inside flour's starch granules at a molecular level, and reversing that same process can make your bread soft again.

Before bread is baked, the starch molecules of the flour stay arranged in a tight crystalline structure. However, as you add heat and water, that structure gets disturbed.

As water bonds with the starches, it pushes the molecules away from each other. When the starch granules expand, they're spread apart into a disorganized, gelatinized consistency.

That space between the molecules makes the bread soft. However, as the bread’s temperature keeps dropping, retrogradation starts and the molecules become tight and orderly again.

That’s because the water fails to keep the molecules away from each other. As a result, the bread's texture gets affected, and it inevitably turns into a food-grade stone.

To reverse the retrogradation, moisten the outside of the bread with water, wrap it tightly in aluminum foil, and put it in a non-preheated oven set to 300° F for 10-15 minutes.

Or, swap the foil for a damp cloth and microwave it at 10 to 15-second intervals. You can also use a steamer basket or a colander set over a pot of boiling water for five minutes.