This is a messy, but fun activity for your kids. I think it’s appropriate from age 2-8 or so. (Then again, I’ve seen teenagers fascinated with the stuff, too, but not for very long.) Mix 1 part cornstarch and 1 part water. I think a cup of each is more than enough for one child. It makes this very strange substance that hardens if you pick it up and roll it in a ball in your hands, but then melts when you take pressure off. It’s also fun to put a spoon in it and tell your child to try and stir it.
It seems messy, but since it’s cornstarch, it just wipes up easily. You can also add a drop of food coloring, but that sometimes dyes the hands.
I tried this with two very active girls, age 5 & 7. They were entertained for an hour, twice.