In most Interactive Learning Environments (ILEs), the human learner interacts with an expert in the domain to be taught. We explored a different approach: the system does not know more than the learner, but learns by interacting with him. A human-computer collaborative learning (HCCL) system includes a micro-world, in which two learners jointly try to solve problems and learn, the human learner and a computerized co-learner. This paper presents the foundations of this artificial co-learner.