My grandmother used to do this trick all the time when I was little. It really helps when you over-bake cookies, leave them out too long at a party, or simply want super soft cookies. You simply stick a piece of bread in an air-tight container with the cookies and overnight you will have deliciously soft cookies. People would look at me like I am crazy, but it works.
This works because the MOIST bread gives up its water vapor into the air, then the DRY bread gains the water from the moist air.
I think I am going to try this trick for stale cereal and chips!