Hmmm... Get's me thinking. What if you have no family, and all your friends are from work at Office? Then you are left with just three balls, which means you could be juggling just one ball (the rubber one) more often.
For most people, their work is part of their spirit, that gets them on their feet, which means the 5th ball merges itself with the 1st ball, Work.
And given that you're under a corporate scheme of Health insurance, and you're part of corporate teams in sports played over weekends, the workout becomes part of your official work; thus adding itself to the only other ball remaining, Work.
So, I think we have a Winner, the Rubber Ball, i.e Work. Let's hope there is no one incompetent enough to drop it to see if it bounces back, or gets tired of holding it.