When bread goes stale, it gets hard. When hard cookies or crunchy cereal go stale, they get soft. Do they both end up with the same amount of softness and hardness? Do they meet in the middle somewhere? What's going on?
D, San Diego
Wait long enough and your cheez puffs and Wonder bread will eventually turn to lumps of damp, furry mold, probably about the same consistency. But since both have a shelf life longer than most good wines, you may have to leave them to your heirs before the full effect is achieved.
When bread goes stale, it's sort of "unbaking" itself. At the molecular level, at least. Lots of the mysterious things that happen in the oven reverse themselves in the breadbox. Just-baked bread is a sort of fragrant, fluffy, unhardened concrete-- a moist mass of gluten (wheat protein) studded with damp, gelatinized starch granules and filled with gas pockets. As the bread cools below 140 degrees, certain molecules inside the starch realign themselves, close ranks, and stiffen, squeezing out the water absorbed during baking. Once the gel around the starch stiffens, the bread is firm enough to cut. This is bread's optimum point. It starts going stale almost immediately.
Starch molecules continue to realign and stiffen, forcing out even more water. Within a week or so, you have a loaf of crumbly, dry starch and gluten covered with a leathery crust that has absorbed the squeezed-out water molecules. The whole process happens much faster in cold temperatures, so don't store bread in the refrigerator. Eat it right away or freeze it. The "stale" taste and smell of elderly bread comes from natural molds beginning to sprout. The industrial solution to all of this is vats of preservatives that slow the migration of water molecules and the growth of molds.
Crispy stuff, on the other hand, has already had the moisture baked or fried out of it. So crisp things don't have the same internal chemistry as a donut or a dinner roll. Sugar will also crispify baked goods. When sugar is heated, it turns to caramel but crystallizes again when it cools. And sugar crystals are "hygroscopic"; they rapidly absorb moisture from the air. So a crisp cookie is like a delicious sponge.