This ended up as more of a science experiment than an art project and didn't really turn out like I expected. The balls didn't bounce as well as a normal bouncy ball and they never fully hardened, but it was fun to do and Mercedes had a great time playing with the goopy balls as they dried.
Supplies: Borax (found in the laundry detergent section of the store)
white school glue
spoons to stir mixtures
2 small plastic containers
pen and tape
zip lock bag
food coloring (optional)
Instructions: Label one cup "borax solution" and the other cup "ball mixture". Poor 2 Tbsp warm water and 1/2 tsp borax powder into cup labeled "borax solution". Stir the mixture to dissolve the borax. Add food coloring if desired.
Poor 1 Tbsp glue into cup labeled "ball mixture". Add 1/2 tsp of the borax solution that you just made and 1 Tbsp cornstarch. Allow the ingredients to interact with each other on their own for 10-15 seconds and then stir them together to fully mix. Once mixture is impossible to stir with a spoon, take it out of the cup and start molding it into a ball with your hands.
The ball will start out sticky and messy, but will solidify as you knead it. Once the ball is less sticky, go ahead and bounce it!
Store the ball in a ziplock bag when you are done playing with it.
Avoid contact with eyes and mouth. Wash your hands and work area when you are finished.