Gelatinous Cubes were originally created by Gary Gygax to poke fun at the fact that the majority of early Dungeons & Dragons maps were made on grids, with 10x10 foot cubes making up a single space. The gelatinous cube, then, is a monster designed specifically to take advantage of this environment, as it is precisely the same size as a space on the map. Rather than actively hunting down prey, the cube makes circuitous "sweeps" of the dungeon it lives in, with its room-filling size allowing it to absorb all organic material it encounters. The traditional method by which it defeats adventurers is to simply engulf them with its mass and suffocate them, then dissolve and digest their remains. Successful cubes can be determined at a glance, as they usually still contain the bones and weapons of previous unlucky adventurers, the bones taking longer to dissolve and the weapons being entirely unpalatable to cubes. The best course of action when facing a gelatinous cube is to duck into a side alcove or adjoining room, as for one, it will ignore the party in favour of continuing its sweeps, and two, cubes always retain their cubic shape, and will not pursue prey anywhere it is too big to fit.
Gelatinous cubes reproduce through budding; when a cube has absorbed a certain amount of material, it will leave behind a smaller cube, which will be able to leave its parent's feeding grounds and find its own, as it is still small enough to fit through the exit. If it does not leave the path, it is likely to be re-absorbed by its parent again, but this just means the parent cube will probably bud again very soon, and since cubes barely count as "alive" (having an intelligence stat of 0), it isn't a great tragedy.