Engineering design was integrated into K-12 science education in the "Next Generation Science Standards" (NGSS Lead States, 2013), but teaching design remains a challenge for educators. Design problems are ill-defined, ill-structured, and complex problem-solving tasks. Their solutions require creativity and recursive, metacognitive processes that cannot be taught with simple algorithms. Moreover, adolescents do not demonstrate fully developed metacognitive skills because they are undergoing profound developmental changes. In this comparative case study, we explored how peer-delivered metacognitive prompts supported adolescents during a design challenge. We investigated how scripted prompts sparked reflection and stimulated design changes and identified which prompts were most effective. We also observed four interaction patterns between paired peers. The interaction patterns influenced the quantity of design changes and shaped the strategies that students used during revisions.