Rational thinking as exemplified in mathematical cognition is of undeniable importance in the modern world. This book documents how a group of three eighth-grade girls developed specific practices typical of such thinking through involvement in an online educational experience. The presentation begins by discussing the methodological approach adopted in analyzing the development of mathematical group cognition. An extended case study then tracks the team of students step by step through its eight-hour-long progression. Concluding sections draw the consequences for the theory of group cognition and for educational practice.