To soften up baked cookies that have hardened after cooling, place them in a tightly lidded plastic container along with a slice of bread. You can just lay the bread on top of the cookies, or put a small piece of foil or plastic wrap on the cookies first. Let the container stayed closed overnight; you’ll find the cookies have softened considerably in the morning, as they will have absorbed moisture from the bread.
Use the bread for toast or croutons, and enjoy cookies that won’t be as hard on your braces. This works best on home-baked cookies, but still improves store-bought types, too.