Left: For the pair of equal-mass tennis balls, the centre of mass or barycentre is located at the centre of the system in the middle of the length of the string, marked by the X. Both tennis balls describe the same orbital path (dashed line). Right: For the pair of unequal-mass tennis balls, the centre of mass or barycentre is located much closer to the more massive, filled ball (coloured red) – marked by the X. This is observed as the more massive ball following a much smaller orbital path/circle whilst the lighter ball follows a much larger orbit/circle.