Can moons have moons?

Planets orbit stars. Moons orbit planets. But no moons have their own moons (let’s call them submoons). Why is that?


A cigar from another star

In Rendezvous with Rama (by Arthur C. Clarke), a mysterious object is discovered passing through the Solar System.  The object has a strange shape — it’s a giant cylinder.  It was discovered by the Spaceguard survey, designed to find objects that might impact Earth (so-called near-Earth objects).  Spoiler alert: the cylinder is a spaceship sent…