Ah, Oobleck...probably one of the most popular labs I do. So popular that the kids - gasp! - actually go home and not only talk about it, but end up making a mess in their momma's kitchens by making some. I have the son of one of our 8th grade teachers in my class and that's apparently what he did after he got home from school on Thursday -fortunately Momma thought it was wonderful.
There really isn't a set recipe for Oobleck, outside of equal parts water and cornstarch. I usually have the kids take a typical plastic spoon and put two heaping spoonfuls of cornstarch in a bowl. I then have them add water a spoonful at a time. Depending on how big their heaping spoonfuls were, it will take 2-4 spoonfuls. It's going to be hard to stir for a bit, but don't add more water. Use your fingers if you have to, and keep mixing in the water. Pretty soon you're going to have Oobleck.
Some people like to add food coloring, but I don't as it's messy enough as it is.