Mix together the water and borax in a medium-sized bowl. It's ok if a little of the borax remains at the bottom of the bowl, but make sure it's mixed as well as possible.
In a separate small bowl mix together the cornstarch, glue, and food coloring. You’ll want to make sure these ingredients are mixed really well for the best results.
Add the glue mixture to the water mixture and stir until the mixture reaches a smooth, homogenous, gooey consistency. This should only take about 10 seconds.
Once it's gooey, use your hands or the back of a spoon to squish it around a bit until it starts to firm slightly. It's ok that it isn’t a ball at this point!
Take the gooey glue mix out of the borax mixture and roll the mixture in your hands until round and hardened.
You can make 1 big bouncy ball or several small ones by breaking bits off before it has hardened. The longer you roll the ball in your hands the harder it will get. If you set the ball down it will flatten a little, if this happens just start rolling it again and it will shape up.