Hasbro's Play-Doh made its debut in 1955 as a spin off from a wallpaper-cleaning compound that was reworked into non-toxic Play-Doh by Kutol Industries. At first, the product dried out very quickly, but in the late '50s the formula was revised so the dough took longer to harden. Sold in cans with resealable lids, Play-Doh will still harden if it is left out too long, but there is a way to revive it.
Separate dried out Play-Doh from other toys or items. Remove any dried, stuck on dough from cookie cutters, tables and other items. Play-Doh comes off of these items easily, but throw away any Play-Doh that has dried with embedded dirt or fibres.
Place the dried dough in a small bowl and add a few drops of water. The amount of water you add depends on how dry the dough is. Add the water a little at a time so you don't over do it.
Knead the dough to incorporate the moisture after each addition of water and allow it to soften. The dough will feel a little grainy---this is the salt that has come to the surface as it dried. It will often leave small crystals on the surface of the dough.
Wet a paper towel and ring out the water. Wrap the clay in the towel and return it to its container with the lid secured. Leave it overnight. This will keep the moisture in the dough and soften it.
Reuse the Play-Doh. After a night in the container the dough should be good as new and ready to use again.
If the original Play-Doh containers get lost or damaged, any container with an airtight lid will work to keep the dough from drying out. There is no way to separate Play-Doh that has been mixed with another colour. Just think of it as tie-dyed. Play-Doh has a lot of salt in it, which can dry out your hands. After use, wash hands thoroughly and moisturise them if they have been in contact with the dough for an extended period.
You can only reconstitute Play-Doh so many times before it is time to throw it away. If your child has a wheat allergy, Play-Doh may give them a reaction. The Play-Doh formula is kept highly guarded but it does contain wheat.