Habitat
Gelatinous cubes are subterranean monsters. There shape allows them to absorb whatever food they can find on floors, walls and ceilings, and their lack of sight makes them ideally adapted to conditions where light is in short supply.
Water and Sustenance
Gelatinous cubes are omnivorous. They feed by engulfing objects and dissolving them within their bodies. They can digest pretty much anything organic; only ores and metals are resistant to this dissolution. Lacking digestive tracts, they cannot excrete or otherwise reject these foreign minerals. Rather, they simply carry these goods around.
Cubes feed indiscriminately, deriving sustenance from living and dead alike, although they exist primarily as scavengers. If no food source is reachable, the gelatinous cube becomes dormant after a week. It can remain in this state indefinitely, becoming active as soon as it receives some kind of nourishment.
Organisation
Gelatinous cubes are not social creatures by any definition of the word. They are scarcely aware of their surroundings, and are unable to distinguish other gelatinous cubes from other life-forms.
Lairs
As they are mindless, have only the most rudimentary awareness of their surroundings and even then perceive them only dimly, they have no territorial instinct. They do not create lairs, and neither do they settle in any area for long. They are simply drawn blindly towards whatever food presents itself, moving along when they have swept their immediate area clean.
Hoards
Gelatinous cubes do not have a hoarding instinct. Any treasure they collect is purely incidental.
Relations with Other Monsters
Gelatinous cubes form an important part of the dungeon ecosystem. Their scavenging keeps dungeon corridors clean. However, to many sages, the cube's ability to fit in is a trifle too convenient to have been determined by purely environmental reasons. Its shape, ideal for sweeping every available surface in a tunnel, suggests that it was engineered; that someone wished to create a creature as capable of scouring dungeon corridors as any ooze, and yet solid enough in its shape to prevent it from sliding under doors. In short, the gelatinous cube was designed to occupy buildings as a kind of mobile cleaning device.