Perusall is a free, web-based tool that allows instructors to create collaborative reading activities for their students based on their text book or any other study materials. Students can highlight text and graphics, ask questions, add comments, or link to other relevant resources. Classmates can see the areas other students have highlighted, read their comments and questions, and respond asynchronously. If a student is interested in knowing the answer to a question posed by another student, they can indicate this by clicking on the question mark icon. When classmates find a student's explanation to be helpful, they can indicate this by clicking on the checkmark icon. A built-in tool, the confusion report, automatically summarizes the top areas of student confusion so that instructors can focus on the content that students are struggling with the most.