Oobleck has always been a hit at my house. It is fun for the kids and very safe. There are just 2 items needed.
Other items you can add is food coloring and glitter but at our house we tend to just go with the natural way so there is no worry of stain anything.
The ratio for mixing is 1.5 parts Cornstarch to 1 part water. Start with 1/2 cup of water and then start slowly adding the Cornstarch.
What can you expect when you play with Oobleck?
If you slowly lower you hand into it, your hand will sink into the Oobleck and your hand will become covered.
If you punch or squeeze the Oobleck, it will feel solid. My kids even used a hammer to smash it to see what would happen. Please use proper safety precautions.
If you make a ball with the Oobleck it will remain in that shape - as long as you keep continuing to make it into the ball shape. When you stop the Oobleck will turn into a liquid and fall through your hands.
Ooleck can be messy when the corn starch dries but it is easily washed away. Playing with it outside is even easier, a hose or bucket of water will clean up the cornstarch.