Orbital evolution of two noninteracting particles and their center-of-mass, initiated without rotation. Over time, both the interior (magenta arrow) and the exterior (orange arrow) particles start to revolve around their mutual center-of-mass (black arrow) in a prograde fashion, as shown by the angular displacement that develops relative to the horizontal dashed line. The quadratic prograde spin-up over time is additionally shown in the inserted panel, with the rotation at the different snapshots depicted by the black dots.