Is this mystifying blend of cornstarch and water a liquid? A solid? Truth be told, it's both! When kids apply pressure -- as when rolling it into a ball -- the mixture magically firms up. But release that pressure, and it will dribble right back out through their fingers. Here's how to easily whip up a batch of your own, because in this case, feeling really is believing.

To Make the Mix
Pour 1 cup of cornstarch into a shallow bowl or pan. Add 3/4 cup of water a little at a time, stirring as you go.

To Play
Show your toddler how to scoop some of the mixture into her hands and then roll it into a ball, much as you would roll a handful of cookie dough. The consistency is key here, so if your magic goo doesn't firm up when you roll it, add a little more cornstarch. Likewise, if it doesn't melt back out of your hands when you release the pressure, add some water, about 1/2 teaspoon at a time.

To Store It
When your child is done playing, leave the mixture sitting out for several hours. Once it's dry, crumble it into a sealed container. Just add water to play again another day.