Humanoid robots will become part of our everyday lives. They have biologically inspired features and psychologically complex properties. How will children interpret these ambiguous objects, discriminating between living and nonliving kinds? Do the biologically and psychologically inspired characteristics affect children's understanding of the robots? How firm is the distinction that children make between living and nonliving objects? To address these questions, 120 children ranging three to five years initially viewed video clips that depicted humanoid robots interacting with a human experimenter on two different dimensions (mobility and, psychologically contingent behavior). The subjects then answered simple questions that probed their animacy judgments and property projections about the robot. The results showed that children's animacy assessments about humanoid robots differed by age. When judging the robot's life status, its mobility was important for four-year-olds and, the psychological contingency for five-year-olds. In terms of the robot's reasoning abilities, the majority of four-year-old children clearly understood biological properties, regardless of the robots' features. However, when reasoning about psychological properties, even five-year-olds occasionally relied on robots' features such as their contingent behaviors. Moreover, the children attributed some but not all animate properties to the robots. Although rent findings show that children possess naïve theories, they do not seem to have a consistent and logical theory of "aliveness," and they apparently develop the concept of a robot by acquiring knowledge about how this boundary object differs from living entities.