Based on our experience of teaching physics in middle and senior secondary school, we have found that students have difficulty in reasoning at the microscopic level. Their reasoning is limited to the observational level so they have problems in developing scientific models of magnetism. Here, we suggest several practical activities and the use of interactive computer simulation software that we've adopted in our physics program to help students conduct model-based reasoning to develop a simplified model of magnetism--the small-magnets model of magnetism. In this simplified scientific model, small magnets representing magnetic domains or atoms align along the same direction, thereby enabling the objects to be magnetic. Using this model, students can more intuitively understand the current scientific models of domain or atomic magnets.