Why does bread go hard while biscuits go soft?

You open the cupboard and find that the sliced bread you bought two days ago is as hard as a brick, while the crackers next to it have gone limp. They were in the same cupboard the whole time.

Why bread goes hard

1. Starch retrogradation. In fresh bread, the starch swelled up with water during baking and its structure is loose. As it cools, the molecules rearrange into tight crystals — and squeeze the water back out.
2. The water leaves. That squeezed-out water slowly evaporates into the air. The inside of the bread dries out, and the texture goes from soft to stiff.
3. It does not reverse. Once those crystals have formed, reheating only partly undoes them. That is why toasting brings back a little softness but never tastes like fresh bread.

Why biscuits go soft

1. Sugar and salt pull in water. A biscuit recipe contains far more sugar and salt than bread does, and both of those grab water molecules out of the air.
2. It starts very dry. A biscuit leaves the factory at about 3% water. That enormous gap in humidity between the biscuit and the air turns it into a water magnet.
3. The structure collapses. As water moves in, the sugar starts to dissolve and the crisp, airy structure glues itself together. The crunch becomes a chew.

In one line: the bread is losing water and the biscuit is gaining it. In one cupboard, two baked things are running the same physical process in opposite directions — because the recipe decides which way the water goes.

What actually helps: freeze bread in a sealed bag. At −18°C starch retrogradation almost stops completely. And drop a sachet of desiccant into the biscuit tin — it can absorb up to 40% of its own weight in water.