In a large bowl or plastic bin, add arrowroot flour, cold water, and food coloring.
Stir with a spoon then by hand. You know you reach the right oobleck texture when it's hard when you apply pressure onto it, crumbly when you grab by hand but runs out like a liquid as soon as you open the hand and release the pressure.
If it's too dry and impossible to gather by hand, add extra water 1 tablespoon at a time. If too liquid and more arrowroot flour.
You can store oobleck in plastic bags or airtight containers for a few days in the fridge. However, the smell turns bad with time, especially if stored at room temperature.
Clean the oobleck with water. It dissolves very well. The hardest part to clean up is the food coloring.