Disclaimer: Our bouncy "ball" turned out more like a bouncy blob, but it was still fun acting like a scientist in the kitchen.
What you need:
Borax (found in laundry section) I went to Whole Foods thinking I would find it there, but they didn't carry it. I actually found Borax at Wal Mart. It's about $3
warm water
corn starch
glue (clear glue makes a see transparent ball and white glue makes an opaque ball)
2 small mixing cups
a stirring stick (plastic spoon)
food coloring (optional)
What you do:
1. Label one cup ‘Borax Solution’ and the other cup ‘Ball Mixture’.
2. Pour 2 tablespoons warm water and 1/2 teaspoon borax powder into the cup labeled ‘Borax Solution’. Stir the mixture to dissolve the borax. Add food coloring, if desired.
3. Pour 1 tablespoon of glue into the cup labeled ‘Ball Mixture’. Add 1/2 teaspoon of the borax solution you just made (I accidentally added Borax the first time and not the Borax solution) and 1 tablespoon of cornstarch. Do not stir. Let the ingredients interact on their own for 10-15 seconds.
4. Allow the ingredients to interact on their own for 10-15 seconds and then stir them together to fully mix. Once the mixture becomes impossible to stir, take it out of the cup and start molding the ball with your hands. The ball will start out sticky and messy, but will solidify as you knead it. 
You can store your plastic ball in a sealed ziploc bag when you are finished playing with it. (The ball tends to flatten out a bit when sitting and does not normally last longer than a couple of days before drying out).
