The best Science, Technology, Engineering, and Mathematics (STEM) design challenges are student centered, with students themselves making the key decisions. But with young children who are still learning basic academic and social skills, implementing projects where they truly take the lead can be quite challenging. To give students at one elementary school the opportunity to drive the process of a STEM design challenge, teachers created a project that paired first graders with third-grade leaders. The goal was for these students to work together to create apps that would be used by the school's kindergartners to learn mathematics concepts. This week-long project not only supported standards of using technology to foster collaboration and creativity (ISTE 2015) but also emphasized the importance of planning as part of the engineering process. Rather than having students passively play games to practice mathematics facts, the teachers empowered students to build their own mobile app games as a learning strategy. The project also advanced the idea of "coherence" related to the Common Core State Standards for Mathematics (CCSSM), in that students created apps to teach mathematics concepts that are common across grade levels (CCSSI 2010). The project is described in this article.