question

Whole books have been written on this. The basic answer for bread concerns the crystals of starch in the flour, which are gelatinized during baking (they take up water and become soft). This starch gradually recrystallises over a few days in a process called retrogradation, so that the bread becomes harder. The starch in cookies also undergoes this process, but it can be overwhelmed by the effect of the sugar that is present in many cookie recipes. Sugar takes up water from the atmosphere, and this is what causes cookies that are left out to go soft. Cake can go either way, depending on the recipe, and both processes can be hugely affected by other ingredients in the recipe.