I remember making this stuff in elementary school. It's just corn starch, water and food coloring. Once mixed, it's hard, but when you pick up a lump of it and let it sit in your hands it turns liquid and oozes. The kids were so surprised by this they couldn't stop giggling. During all the playing we talked about solids and liquids. The kids and I had a great time naming something and then determining whether it's a solid or liquid.
I know this Gak stuff looks super messy, but the great thing is it's hard on the table so you just pick it up and put it back in the bowl. Clean up is so easy too. The Gak washed right off their hands (added bonus: the corn starch made them super soft) and quick wipe of the table and you'd never know the mess that had been there minutes before.