I was introduced to gak by my science teacher in junior high. Cornstarch in water can't decide whether it is a liquid or solid. You can pick up balls of it, but it immediately melts through your fingers. In junior high our teacher let us shatter it with a hammer then watch it melt. I don't let Helen use hammers, but she does get out play spoons and spatulas.
To make gak, mix some cornstarch with a small amount of water. It is about 1/2 cup cornstarch to 1/4 cup water. I think. I never measure. I know I have the right consistency when it feels like I am stirring a solid, but looks like I am stirring a liquid. Then add some food coloring.
I though gak would be shelf stable in an air-tight container. I was wrong - it is a science class petri dish. Just throw it away when you're done.
1/23/10 Update - Here are some more photos of another time we played with gak.