Mix approximately one cup of corn starch with approximately one-half cup of water. Do this in a bowl, simply using your hands to slowly mix it all up. As you mix, the mixture will feel kind of gooey or pasty. Grab a small handful of the goop, and roll it between hour hands... it acts like clay or putty. Let the blob sit in your palm, and it runs like a liquid. Squeeze it and it acts like a solid again.
Cornstarch molecules are relative long and large. When you slowly stir them, or just let them be, they are able to slip past each other, and flow like a liquid. If you press on them (such as when you roll the mixture between your hands), you are trying to move the molecules past each other more quickly than they normally can go. They get "tangled up" in each other, essentially, so they don't flow. In other words, they act more like a solid.