What separates your mind from an animal's? Maybe you think it's your ability to design tools, your sense of self, or your grasp of past and future--all traits that have helped us define ourselves as the planet's preeminent species. But in recent decades, these claims have eroded, or even been disproven outright, by a revolution in the study of animal cognition. Take the way octopuses use coconut shells as tools; elephants that classify humans by...