A typical loaf of bread will last for several days before it gets hard and stale. At this rate, some are inclined to throw it away; but Rochelle Billow of Bon Appétit says there’s still hope.
H2O to Rehydrate Stale Bread
Assuming there are still no signs of mold growth, a slice of stale bread will benefit from a quick bath. Make sure the crust is positioned in a way that the cut-side is facing away from the faucet, but a soaked slice is still good. Warm it up for around 7 minutes (add 3 to 5 minutes if you really drenched it) in 300 to 325 degrees in the oven.
Bread goes stale due to a process called retrogradation. As starch molecules harden over time, the bread loses its moisture, which gives its softness. Reheating the bread forces the molecules to reabsorb the moisture, further enhanced by water.