Cornstarch and water combine to form a non-Newtonian fluid. In this case, instead of having a constant viscosity, it moves like a liquid under a small amount of force and acts like a solid if a large force is applied.
It's a good excuse to make a mess and is a lot of fun to play with!
At first the kids enjoyed just getting gooey. They really noticed the response of the mixture when they dripped a bit on the counter and saw that they could smear it if they were gentle, but if they used a lot of force it held its shape. That led to a lot of goop on the counter!
Eventually the water started to evaporate from our mixture. At that point they had fun scooping the powder into the bowl again and adding water to it. They spent quite a while pouring it into different cups and smooshing it around.The kitchen was a big mess after their experimenting, but it was very easy to clean everything up. The cornstarch dissolved right off of them when it came time to wash up and the goop on the counter and floor dried very quickly and could be swept away.
We used a ratio of 1 cup cornstarch to half a cup of water.
The video I link below shows people running across a pool filled with cornstarch and water - how fun!