This is a classic concoction from the play dough family. It is neither a solid nor a liquid...but a colloid.
You just mix one cup cornstarch with 1/2 cup of water. You mess with the ratio until the glop feels just right.
I did this with my 4 year old son. Before playing with the glop, we talked about liquids, solids and gases and found examples of each in our kitchen. Then we played with the glop, glurch...whatever you choose to call it, and about it's properties that are like a liquid and others that qualify it as a solid.
Have fuN!