rotating bodies

Hi, Paul, me again; you know, the 3rd grade teacher:

I explained your answer to my students regarding what would happen if the earth stopped rotating, and did a brief demonstration connecting it to the 1st law of motion, and several "got it." However, the info, of course created a new question: Why do planets rotate anyway? What started their motion, and lacking a force to continue adding energy to their motion, why don't they eventually slow down and stop moving like a rolling ball does?

Thanks a bunch, Pat