The difference between a cake and a biscuit (or a "cookie" if you're American) is that when they go stale, cakes go hard and biscuits go soft. We've all heard the famous story of McVities proving that the Jaffa Cake is indeed a cake by demonstrating that they go hard if left out. But why does this happen in the first place? When you bake a cake or a loaf of bread, the starch granules in the flour burst and start to absorb water, making the structure soft and pliant. However, over time the granules dry out and begin to recrystallise, resulting in the hard loaf of stale bread. Of course, biscuits contain flour too, but they also contain large amounts of sugar. Sugar absorbs the moisture from the air over time and causes then to go soft. Of course, if you remove the moisture from the air, your biscuits will remain crisp. What the two baked goods are doing is equalising their moisture content with their surroundings. A cake has a higher water content than the air, so the moisture escapes, whereas the biscuits are baked in such a way that the moisture is driven off to make them crisp, so the balance is redressed as they absorb the moisture around them.
