So-called mirror neurons, discovered in monkeys, fire both when a person performs an action and when a person sees someone else perform the same action. These brain cells may be enabling us to understand the intention behind other people's behavior. To test the idea, investigators from the University of California at Los Angeles showed 23 volunteers three sets of video clips. The first depicts tea and cookies either arranged neatly or haphazardly: the context for an action. The second shows someone grasping a lone teacup: the action. The third shows someone grasping the teacup amid the other objects, implying the actor's intention of either having a sip of tea or cleaning the teacup. A specific group of mirror neurons in the right inferior frontal cortex was active only during the videos depicting intention, the researchers report online in the March PLoS Biology.