Security in mobile ad hoc networks is difficult to achieve, because of the dynamic topologies, limited resources, the absence of a certification authority and the lack of a centralized monitoring point [1]. Most of the attacks in MANETs are routing protocol attacks. The sinkhole effect is caused by attempts to draw all network traffic to malicious nodes that broadcast fake shortest path routing information. The sinkhole nodes should be detected and detached as early as possible. The detection mechanism should also guarantee higher detection rate and lower cross over error rate. In this paper we propose a novel technique to detect the sinkholes which uses the mutual understanding among the mobile nodes.