Reference is a pervasive phenomenon in natural language, and it has been addressed by many researchers. Previous focusing approaches based on a local focus fail to resolve some references due to either frame-selection problem or multi-frame focus problem. Further researches on topic focusing in reference resolution pointed out that the discourse topic should always be in consideration rather than a local focus in the resolution process. Therefore, a new approach, which is called Charniak's marker-passing algorithm, is presented in this paper. This algorithm is carried out based on the premise that the most active concept which is acceptable syntactically and semantically as a referent is the referent. By defining the activeness of each concept and propagating activeness to related concepts, the discourse topic can be always considered. This paper presents an educational tool for Charniak's marker-passing algorithm. Although this model has already been implemented on SNAP (Semantic Network Array Processor) simulator, this paper is focus on the graphic demonstrations of the resolution procedures for educational purpose only. It shows more than 92% success rate in pronoun resolutions of simple sentences without subordinate clauses.