To make Oobleck, you’ll need to pour some cornstarch into a big bowl. Use at least one pound. Run your hands through it so you can feel its texture. It feels different from flour. Start adding water gradually, and mix in the water with your hands. Keep adding water and mixing it until you have a very thick paste. You’ll end up using about a half cup of water for each cup of cornstarch. Now you get to experiment with it. Can you make a ball of it? What happens when you crack the ball in half? Can you make it ask like both a liquid and a solid? Can someone run across a bathtub full of it? (hint: check YouTube.)
When it’s time to clean up, put as much of it as possible in a plastic bag, tie the bag, and throw it in the garbage. Pouring a lot of Oobleck down the sink might clog the drain. I don’t recommend trying to save the Oobleck for another day, since it can get moldy. Just get more cornstarch, and make more Oobleck.