This Instructional Activity is a part of the Lesson 3, Unit 8, Grade 8. This activity is the second of three activities in which students investigate the value of square root of 2. As a result of the previous activity, students should believe that square root of 2 is about 1.5, maybe a little bit less. The goal of this activity is to achieve a more precise value of square root of 2 than "about 1.5, maybe a little less." In other words, an exact solution to x^2=2 is required. In this activity, students will consider whether certain candidates are solutions to this equation. In the whole-class discussion that follows, rational number is defined.