Measuring the minds of other creatures is a perplexing problem. One yardstick scientists use is brain size, since humans have big brains. But size doesn�t always match smarts. As is well known in electronics, anything can be miniaturized. Small brain size was the evidence once used to argue that birds were stupid�before some birds were proven intelligent enough to compose music, invent dance steps, ask questions, and do mathematics.Octopuses have the largest brains of any invertebrate. On average the size of a walnut�as big as the brain of the famous African gray parrot, Alex, who learned to use more than one hundred spoken words meaningfully. That�s proportionally bigger than the brains of most of the largest dinosaurs. For its color palette, the octopus uses three layers of...