One evening we neglected to do the usual kitchen clean-up.  I discovered in the morning that although a bread stick had become so hard I could use it as a hammer, our cookies had become soft and unappetizing.

Cookies and bread differ in both composition and texture.  Cookies are denser, and contain much more sugar than bread does.  Cookies become soft because finely powdered sugar is what we call hydroscopic:  that is, it soaks up moisture from the atmosphere.  The dense texture of the cookie helps retain and distribute that adsorbed moisture by capillary action.  The denser the cookie is, and the more sugar it contains, the more likely it is to go soggy.

Bread has little or no sugar to attract water vapour.  Bread also has a more open structure, and so there is little capillary action.  However, there is a much greater surface from which moisture can be lost.  The reason bread becomes harder when it goes stale is because the starch molecules move together in a tighter, more ordered arrangement when the moisture which separates them is removed.

You can try your own experiments by putting some open textured biscotti, which is not very sweet, with some crispy sweet cookies in a sealed container.  The cookies with high sugar content will gain weight by adsorbing water, whereas the biscotti with no sweetener will lose weight.

So next time you want crisp cookies and soft bread remember to store them properly.