The word modeling is becoming more and more common in physics, chemistry, and general science instruction. In physics, students learn models of the solar system, light, and atom. In biology courses they encounter models of joints, the circulatory system, and metabolic processes. The benefits of engaging students in model building are described in the literature. "Modeling instruction" is an example of a whole curriculum based on the idea of modeling. However, in a traditional physics class students do not have a clear understanding of what the word model means, and thus do not appreciate the role of this notion in physics. Physics teachers also have difficulties defining this word. The purposes of this paper are (a) to reexamine the word model as it is used in science, and (b) to suggest several types of tasks that engage students in the construction of models in a regular-format introductory physics course.