|Questions and answers boards are popular among enterprises. A dedicated question and answer site called Piazza is in widespread use in the educational domain. We present the design of an educational Q & A system called Green Dolphin and report on a controlled study of its usage and impact in a graduate level Computer Science course. In particular, we implemented a delay in the publication of the instructor's answers to questions in order to encourage students to answer questions from their peers. We also implemented a technique to automatically identify "expert" students from their question asking and answering behaviors, and a point-based reward system for asking and answering questions. We compared Green Dolphin usage patterns with those of Piazza. We found that delaying instructor answers increased students’ participation, the reward system motivated students, and students felt pride in being identified by Green Dolphin as experts.