How do you roll a ball from the inside?First, let’s look at the bottom of BB-8. It’s a sphere that can roll. So, how do you make a sphere roll without pushing it? This one isn’t so difficult. All you need is a moveable mass inside the sphere. Maybe this mass is a tiny car with wheels (on the inside of the sphere) or maybe it’s a hamster. Either way, when the mass moves up the wall of the sphere a little bit, the center of mass for the whole sphere shifts.