Although young people in 50 U.S. states and 21 countries learn about water resources through Project WET (Water Education for Teachers), few researchers have conducted summative evaluations of the program. The authors employed a partitioned, or differential, treatments design in which two groups of 6th-grade students received overlapping but unique lesson components. Using hierarchical linear modeling, the authors found that classrooms from both groups had similar pre- to posttest gains on a test of the common material, but each group outperformed the other group on tests of the unique material a group experienced. (Contains 3 tables.)