Understanding sources is key to developing evidence-based arguments in social studies. The Bookmark tool supports students as they read, analyze, and evaluate sources through a series of prompts to guide annotation and thinking. The Bookmark has three sections: Part 1 focuses students on noticing the information about the creation of a source, Part II focuses on the meaning of language in the source, and Part III focuses on evaluating the usefulness and credibility of the source given the central question of the investigation. These steps guide students from noticing features of the source and its language to drawing inferences and conclusions about the source. Students will use the annotations they make on each source later, as they weigh evidence across sources, and plan and compose their arguments.