In another cup mix the glue, cornstarch, and food coloring.
Pour the glue mixture into the water-borax cup.
The glue mixture will harden after 10 seconds; use a fork to take it out of the water. If the glue mixture is still sticky, squish it with your hands and dip it back in the water.
Roll the mixture in your hands to make a ball. The more you handle the mixture the firmer it will become.
You are done!
Keep in mind that the balls will flatten a bit after sitting for a while. Store them in small plastic containers or a plastic bag. If they get flattened roll them between your palms again, this will bring them back to the ball shape.