Oobleck is a wonderful and very messy activity. You can grab it fast and it seems solid, then let it turn into a liquid and drip between your fingers. In a warm climate like ours, we let it dry outside and rehydrate it the next day. In cooler areas it might mold if you leave it out.
It has always been a dream of mine to find a source for past-its-expiration-date cornstarch so that we can fill a small kiddy pool with Oobleck and walk on it! A girl can dream.
1 Cup water
2 Cups cornstarch
a few drops food coloring
Mix together and play!