In this first module of Grade 2, students make significant progress towards fluency with sums and differences within 20 (2.OA.2). Fluency, coupled with a fundamental grasp of place value, rests on three essential skills: 1) knowing number bonds of ten, 2) adding ten and some ones, and 3) the number bonds (pairs) of numbers through ten. Topic A energetically revisits this familiar ground from kindergarten (K.OA.3) and Grade 1 (1.OA.6) at a new pace; we move quickly from concrete to pictorial to abstract. All the material included herein can be included in daily fluency work, and should be if students lack fluency with mental strategies. In Lesson 1, students use ten-frames to model number bonds of ten as they generate addition and subtraction number sentences and solve for the missing part by bonding, counting on, or subtracting. Students record and share number bonds of 10 to regain their Grade 1 fluency and understanding. Lesson 2 continues with students revisiting number pairs through 10 and each pair’s related facts. Again, students work with ten-frame cards to create number bonds and to determine a corresponding subtraction number sentence. As students play a part–whole game, they practice finding the missing part and decomposing a given quantity in a variety of ways. The application problems in these earlier lessons follow the concept development to provide students with the opportunity to discover the connection between the one-step story problems (2.OA.1) and the models (i.e., ten-frames, number bonds) and to articulate their observations with classmates.