In many university mathematics courses, homework accounts for the majority of students' interaction with mathematics content. However, we know little about students' activity as they complete homework. This paper presents an empirically-based model of students' activity as they complete an online homework assignment. I developed the model based on analyses of video recordings of nine Calculus II students completing an online homework assignment and follow-up interviews with the students about the homework session. In the context of the introduced model, I present two additional findings. First, students' activity when solving online homework problems is cyclic and similar to mathematicians' activity when problem solving. The online platform contributes to this by verifying answers and providing students multiple tries per problem. Second, students leverage their multiple tries per question and ability to submit parts of questions individually to obtain intermediate feedback. They use this feedback as formative assessment to guide their work on the remainder of the problem.