Cover the work area with an old plastic tablecloth to keep it clean and free of glue. Set two recyclable plastic cups atop the tablecloth.
Measure and pour about 1 tablespoon of school glue into one of the plastic cups; the amount doesn't have to be exact. Squirt a few drops of food coloring into the glue, stirring with a craft stick or coffee stirrer until the colorant mixes into the glue.
Pour 4 tablespoons of water into the second cup and stir in 1/2 teaspoon borax with a craft stick until it dissolves.
Add 3 tablespoons cornstarch to the borax mixture and stir it briskly with the craft stick until the cornstarch dissolves.
Pour the watery mixture into the cup containing the glue, stirring with a craft stick until the mixture clumps together and becomes difficult to stir. When you're no longer able to stir it easily, fish the rubbery blob out with your fingers.
Roll the gooey material between your palms, shaping it into a sphere. It may feel sticky at first, but it eventually will lose the stickiness. Wipe your hands on a paper towel or damp paper towel occasionally as you roll the mass to make your hands less sticky. Continue rolling the substance until it doesn't feel sticky and is as round as you can make it. The ball is ready to bounce at this stage.