Developmental psychology experiments on tool use show that infants’ capacity to use a rake-like tool to retrieve a toy arises quite suddenly around 18 months. We use a developmental-robotics model to propose and test two alternative hypotheses to explain this conundrum. Both hypotheses rely on the assumptions that tool use involves goal-directed behavior processes guided by the goal of retrieving the toy, and that “understanding how to use a tool” means acquiring the capacity to assemble a sequence of actions to accomplish the goal (e.g., to “hook” and then “retrieve” the toy). The first hypothesis is that the tool-use ability emerges when the infant develops enough planning capabilities. The second hypothesis is that the ability emerges when the infant’s intrinsic motivation system develops and makes playing with a couple of objects interesting enough so that the infant plays with objects similar to the tool at home and thus acquires the actions needed to retrieve the toy in the lab. These hypotheses are tested through a neural-network architecture controlling a simulated humanoid robot tested with the tool-rake task. Given the assumptions made in the model, the results show that both hypotheses can reproduce the average behavior of infants but only the intrinsic-motivation hypothesis can reproduce the sudden tool-use improvement.