We argue that beyond metaphors, according to which learning is a process of knowledge acquisition by individual learners (a "monological" approach) or participation to social interaction (a "dialogical" approach), one should distinguish a "trialogical" approach, i.e., learning as a process of knowledge creation which concentrates on mediated processes where common objects of activity are developed collaboratively. The third metaphor helps us to elicit and understand processes of knowledge advancement that are important in a knowledge society. We review three approaches to knowledge-creation, i.e., Bereiter's knowledge-building, Engeström's expansive learning, and Nonaka and Takeuchi's organizational knowledge-creation. We give a concise analysis of the trialogical character of the knowledge-creation approach, and illustrate how the third metaphor may be applied at the school level. (Sami Paavola and Kai Hakkarainen)