This article aims to share five practical approaches for how teachers can identify, adapt, and create their own writing prompts they assign to students: (1) Promote Students' Solution Paths; (2) Go beyond Asking Students to Simply "Explain"; (3) Prompt Students to Share Their Reasoning; (4) Have Students Consider the Validity of a Given Solution; and (5) Have Students Debate the Validity of Two Given Solutions. The authors present the purposes of the approaches, provide examples and nonexamples of each approach across grade levels, share samples of student work, and discuss how student writing can augment teachers' formative assessment process to realize the depth of their students' thinking more comprehensively. Before doing so, the authors define "mathematical thinking" to reflect the kind of responses they have sought to elicit from students through written prompts and inform decisions teachers can make when applying the five approaches to their contexts.