All physical bodies have a center of mass, and sometimes several. These are points within their volumes at which the mass of the body acts as though it is concentrated in a single point in space. A sphere has a single one at its geometric center. More complex shapes have multiple points. This means that every body already has a 'center' to it about which it would tend to move if properly stimulated to do so. This can be done in any of a zillion ways, with the net result that virtually any physical encounter between real bodies is almost guaranteed to set them rotating about one of their centers of mass. In fact, rotation is the natural and common state of any body.