Most of the existing clock synchronization algorithms for wireless sensor networks can be viewed as proactive clock synchronization since they require nodes to periodically synchronize their clock to a reference node regardless of whether they use time information or not. However, the proactive approach wastes unnecessary energy and bandwidth when nodes don't use time information for their operations. In this paper, we propose a new clock synchronization scheme called Reactive Clock Synchronization (RCS) that can be carried out on demand. The main idea is that a source node initiates a synchronization process in parallel with a data communication. To propagate clock information only when there is traffic, we embed the synchronization process in a data communication process. The results from detailed simulations confirm that RCS consumes only less than 1 percent of the energy consumption compared to two representative existing algorithms while it improves the clock accuracy by up to 75.8%.