The field of intelligent agents is one of the most important fields of Artificial Intelligence. In this paper, we propose a multi-agent system for an educational application. The system includes an explainer module, a problem solving module and an evaluation module. The system has a graphical user interface and can be connected to the Internet. We begin this paper with a survey about the intelligent agents, then we describe a part of this system which is the explainer. The explainer is an important module in the system which shows the domain knowledge in a graphical form with animation. The student can go through all the lessons of the domain knowledge which is in a data base form. The domain knowledge consists of a part of the mathematics course of grade four in elementary school. A case study is demonstrated for the lessons of factors and prime numbers, then number factorization