Cornflour Slime Oobleck!
We discovered oobleck has properties like a liquid. It can pour off a spoon and flow through our fingers. It also has properties like a solid when you hit it with a force it holds its shape. If you try to quickly push your finger in it it feels like a solid but if you slowly push your finger through it flows like a liquid.
We found 2 cups of cornflour to one water made the best ooblek.