Grown-ups and preschool children consider internal characteristics to be of more importance than external characteristics when identifying an agent over a span of time. The authors looked into how this idea originally developed by means of a hands-on differentiation experiment. The participants were 13-month-olds. They looked at somewhat sheer objects similar to animate agents placed inside a box. This box was within reach of the infants, however, they were unable to see its interior. In the trials, objects were put inside the box. The experiment showed that infants depend more on the interior than the exterior characteristics of the objects when deciding upon the number of agents taking part in an event. In another experiment, it was shown that this outcome was particularly true for animate beings. The findings of this study provide evidence that, over a span of time, infants are more likely to decide on an agent’s identity by using internal characteristics rather than relying on external characteristics of objects.