We investigate thirty-three second and fifth-grade students' solution strategies on integer addition problems before and after analyzing contrasting cases with integer addition and participating in a lesson on integers. The students took a pretest, participated in two small group sessions and a short lesson, and took a posttest. Even though the results reveal significant gains for both grades from pretest to posttest, second graders gained significantly higher than fifth graders. In this paper, we explore students' treatment of the negative sign and describe this gain difference. [For complete proceedings, see ED581294.]