SOS is a distributed mobile question and answer system based on social networks that leverages lightweight knowledge engineering techniques. It enables mobile users to forward questions to potential answerers in their friend lists in a decentralized manner for a number of hops before resorting to a server. This reduces costs compared to centralized systems by avoiding high server loads and bandwidth usage. The system was tested through simulation and deployment at Clemson University, showing high query precision and response times with low overhead.