If you need to store cookies or pastries for a few days in a tupperware or other type of container, to prevent them from getting dry and brittle, put a piece of bread in the container right along with the cookies. It will keep them very soft and moist. Sometimes, if you burn the cookies a little and they seem very tough and crunchy, leaving them in a container with the bread over night will actually soften them up and make them better than when they came out of the oven! via
WHY? It’s caused by osmosis. The moisture in the bread (which is a moister environment) moves into the cookies (a dryer environment) until it reaches equilibrium (same amount of moisture in both environments) via
Do It And How 