Why bread goes stale
Bread goes stale because its starch slowly rearranges and water moves away from the soft crumb, making it firmer and less pleasant to eat.
It's often said that putting bread in the fridge keeps it fresh — in fact, cold storage can make the crumb firm faster, even though it may help delay mold.
Staling is a change in texture, not simply bread losing all its moisture, which is why bread can become firm even inside a closed bag.
Think of fresh bread as a soft sponge holding water in just the right places; over time, the sponge reshapes and the water shifts, so the bread feels hard and dry.
Understanding the process helps you store bread better and avoid confusing ordinary staleness with mold, which is a separate problem.
Suppose you will eat half a loaf tomorrow and save the rest for next week: keep tomorrow's portion sealed at room temperature and freeze the rest, because freezing slows the changes that make bread firm. When you thaw the saved portion, it can regain a pleasant texture, while warming a stale slice may soften it only for a short time.
Starch crystals create firmness
After baking, starch molecules gradually line up into more ordered structures that make the crumb feel tougher.
The crust and crumb trade moisture
Water moves between the crumb and crust until their moisture levels become more similar, often softening the crust while drying the inside.
Heat can briefly reverse it
Warming stale bread loosens some of the starch structure and redistributes moisture, but the effect fades as the bread cools.