Bright enough to do the moving-rock trick.

The earliest cephalopods, which lived about a half-billion years ago, had shells. Over the next 250 million years, they evolved into giant predators. They shot bursts of water out of siphons to swim—a prehistoric form of jet propulsion. * But their glory was cut short by fish with jaws—our ancestors. Fish could swim faster by bending their bodies than cephalopods could move by jetting. Today, only a single shelled cephalopod survives—the nautilus, which spends most of its life lurking deep underwater.

The other living cephalopods lost their shells. While they gave up a defense against predators, they were free to evolve new skills. Squids became fast swimmers. Octopuses instead moved to the sea floor, where they could use their shell-free bodies to explore cracks and crevices for prey. But in order to survive in this new niche, they had to become fast learners.

Jean Boal and her colleagues have done some experiments that show how good octopuses are at learning geography. Boal put the octopuses in tanks with an assortment of landmarks, such as plastic jugs, plates of pebbles, and clumps of algae. It took only a few trials for the octopuses to find the quickest route to a hidden exit in the bottom of the tank. What made Boal's results particularly impressive is that the octopuses were learning two completely different mazes at once. Boal would move them from one to the other after each trial. Somehow, the octopuses could keep track of two geographies concurrently. When octopuses are moving across new terrain, they can perhaps learn the best escape from predators.

Octopuses escape from predators not just by hiding quickly but by deceit. One of the most impressive examples of this deception is what marine biologist Roger Hanlon calls the moving-rock trick. An octopus morphs into the shape of a rock and then inches across an open space. Even though it's in plain view, predators don't attack it. They can't detect its motion because the octopus matches its speed to the motion of the light in the surrounding water.

For Hanlon, what makes this kind of behavior remarkable is that it's a creative combination of lots of behaviors, used to address a new situation. Similarly, when an octopus escapes an attack, it may puff up its body and turn white to scare a predator, shoot off puffs of ink to distract it, zigzag through the water, and then suddenly switch its skin to match the surrounding coral.

There's not much point in trying to pin this sort of behavior to some human-based scale of intelligence, because our behavior emerged as apes adapted to life spent on two legs, in groups, and using our hands to make tools. We'd fail pretty badly at an octopus-based test of intelligence, but surely we wouldn't hold it against ourselves.

Correction, June 25, 2008: The original version said that early cephalopods used bursts of air to propel themselves, and that octopuses used jets of water to push toys around a tank. In both cases, the animals used water, not air. (Return  to the corrected sentence.) The original version also said the octopus has "half a million" neurons instead of "half a billion." (Return  to the corrected sentence.)