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Clock skew is defined as the rate of deviation of a device clock from the true time. The frequency of a device's clock actually depends on its environment, such as the temperature and humidity, as well as the type of crystal. The main contributions of this paper are two-fold. First, we experimentally validate that MICAz and TelosB sensor motes have different and unique clock skews. Furthermore, the clock skew of a node can easily be monitored, even via a multi-hop Wireless Sensor Network (WSN). We argue that this feature can be used for identification of the nodes, detection of Wormhole and Sybil attacks. Second, we show that the clock skew of a sensor node varies with the temperature. We explain how this property can be used to detect malicious and mal-functioning nodes and to geo-localize them.