I tried the messy, tiring, and time-consuming kneading method and the not quite effective leave-it-damp-in-the-container method. After months of tinkering, I have discovered the best and easiest way to restore dry Play-Doh to its perfect state (besides Hasbro's former suggestion that you buy a new can). Here's what you do:
1. Break the hard Play-Doh up into pieces the size of shelled peas and put them into a one-quart Ziploc bag.
2. Sprinkle some water in, enough to get all the pieces damp but not enough to leave a lot of excess water. Seal the bag.
3. After a few minutes, smoosh all of the Play-Doh into one corner of the bag. Let it sit this way overnight.
4. Open the bag in the morning and hand the Play-Doh to a delighted toddler. It's as good as new! (And then rinse the bag for reuse.)
If you liked this, you may enjoy some of my other household hints: how to unshrink a wool sweater, how to make tator tot hotdish, how to make the world's best pancakes, and how to slow-poach eggs. Look out, Heloise!