This dissertation study explores the emergence of off-task engagement, in which three Chinese first-graders write mathematically. It seeks the solution to the conflict between the intended mathematics writing and the realized mathematics writing, which ties to the Cartesian rationality dominating mathematics classrooms--that tends to ignore children's genuine mathematical thinking in writing. To make off-task mathematical writing comprehensible, I experiment with Deleuze and Guattari's ontology in coordination with videotaped recording. What I have learned from the analysis is that children's off-task writing as a non-normative way of written communication displays nonsense-making in terms of writing in mathematics, by drawing collective forces distributed across the inside and outside of the mathematics classroom. To be more specific, it present (1) the contingency of frivolous gestures, postures, and sounds emerges in the child-textbook-mathematics interactions, bringing to the fore the becoming of mathematics writing tied to the indeterminate dimension of a mathematical concept; (2) off-task mathematical writing reinscribes a child's affective and visceral reactions to the writing environment in the process of becoming mathematics without predefined progression; (3) a child's writing in disarray unfolds a process of modifying behavior patterns to serve the expressive function of his mathematical writing by organizing its inner drive and its environmental variables. This study joins the conversation on challenging Cartesian onto-epistemology that regulates how children should write in mathematics classrooms and how we should know about children's writing in mathematics. The study highlights the creation unfolding in children's engagement into mathematics writing, where the off-task and the on-task converge, the non-mathematical and the mathematical meet, and the signified and the non-signified encounter. Besides, the study emphasizes teachers' and researchers' examination into mathematics writing in terms of a relational field, attending to the ongoingness of a process rather than relying on the already-existing measurable or descriptive units. In addition, the study suggests that the attention to relation and process is a way of democratic practices to overcome the masculinist bias in understanding off-task writers in math class. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]