Stop and think questions and challenges are sprinkled throughout the text to provoke thought while students are reading, to support teaching assistants or others who are conducting discussion sections, and to provide supplementary homework or in-class writing assignments. For easy reference, each of the more than 380 Stop and think's is individually numbered within each chapter. They are designed to help students learn to read mathematics the way mathematicians do, with pencil in hand and with ``brain open'', as Paul Erdös might have said.