Hyperion, a moon of the planet Saturn, is really very funny. It is the only moon in the Solar System that rotates chaotically:). Basically, it tumbles around in space. This is because the moon looks like a irregular pile of rubble, which suggests that it is a fragment of a larger body that was broken by a large impact in the distant past.
Cassini-Huygens is a joint NASA/ESA/ASI unmanned space mission intended to study Saturn and its moons. It is currently surveying this tumbling moon, Hyperion. The Cassini spacecraft has also captured a video of the moon, which can be seen here:).

Hyperion (Courtesy: Wikipedia)Preliminary estimates of its density show that Hyperion is only about 60 percent as dense as solid water ice, indicating that much of its interior (40 percent or more) must be empty space. This makes the moon more like an icy rubble pile than a solid body. The moon’s spongy-looking exterior is an interesting coincidence, as much of Hyperion's interior appears to consist of voids. Hyperion is close to the size limit where, like a child compacting a snowball, internal pressure due to the moon’s own gravity will begin to crush weak materials like ice, closing pore spaces and eventually creating a more nearly spherical shape.
So in Hyperion, we see the maximum size a pile of rubble can take, beyond which the rubble compactifies under its own mass, and slowly forms a sphere:D.