Failure to understand could occur for a variety of reasons. There may be a failure to construct an adequate, coherent mental representation of the information in a situation. The learner may simply not know understanding is expected and so does not attend in a way that makes it possible. He or she may be unable to construct a mental representation because of a lack of relevant prior knowledge, excessive mental demand, not knowing what is relevant, or failing to notice relevant relationships within the new information and between it and prior knowledge. There could be a failure to manipulate a mental representation or relate it to others because of a lack of ‘rules’ for governing the relationship. There could even be a failure to recognize a mental representation as being the state that was to be understood or achieved, as when a solution to a problem is found but is not recognized as such.