Add 1 cup of water to a measuring cup then add the green food coloring. I start with 5 drops and if you want it darker add a few more. Mix it with a spoon until fully colored.
In a large bowl or rectangular container combine the cornstarch and colored water.
At first, it will be thick but slowly add more water if needed. I think it is easier- and more fun- to mix it up with your hands!
Now you have ooey gooey, oh so fun, Oobleck to play with!
Notes
You can store in a resealable bag or container. Add a little water if it gets dried out.