Pour water into a mixing bowl and add 5-10 drops of food colouring.
Add 1 cup of cornstarch and stir until combined.
Add a second cup of cornstarch and stir until combined, using your hands if needed.
Check the consistency of the oobleck. It should behave as a liquid when resting, and as a solid when pressure is applied to it. If mixture is too runny, add more cornstarch; if it is too solid, add more water. Then, give it to your child to play with!
Notes
Cornstarch can be replaced with another starchy flour like arrowroot flour, tapioca flour or potato starch. To clean up oobleck, leave it out to dry overnight and dispose of it by throwing it in the trash or compost.