After all that perfect chocolate chip cookie baking, what's the use when your batch goes stale in a matter of days? Cookies are great out of the oven, but biting into a hard crumbly mass later in the week is no fun. That's why you need to understand the science of cookie osmosis, or How to Keep Cookies Fresh.
The trick is simple: place a slice of fresh bread in with your cookies a day or two after you've baked them, or whenever you find their texture has deteriorated. The moisture from the fresh bread will migrate to your cookies (through cookie osmosis, see diagram above), rendering them soft and chewable again. It will literally unstaleify them!
Special thanks to my mother-in-law Dee, who passed on this technique to her son, who introduced it to me.