Oobleck is a non-newtonian fluid. This means that it can flow through your fingers like a liquid, but becomes hard when sudden force is applied. it's super easy to make and only uses 2 cheap ingredients.
Step 1: Materials
You will need to get...
1. Cornstarch (will thicken the mixture)
3. A bowl or other container
4. A measuring cup
Step 2: Making Oobleck
The actual making of the oobleck is simple. All you need to do is add 2 parts cornstarch to 1 part water. 2 parts cornstarch to 1 part water means that you add twice as much cornstarch as you do water.
Step 3: Playing With Oobleck
When I make oobleck, I like to hit it as hard as I can to see it harden. If you spread some out on a flat surface, you can hit it with a hammer to see how hard you can get it.
Step 4: Clean Up & Drying Out
Oobleck is really messy, so you'll probably want to clean up afterwards. I like to use a damp paper towel, then a dry paper towel, and then another damp paper towel. It's most convenient to play with your oobleck outside though, because you can just hose your surface off afterwards.
Your oobleck will dry out if you leave it overnight, but you can always re-hydrate it. Just add more water until it gets to a consistency that you like.