The Spanish Inquisition Protocol (SIP) reduces Wireless Sensor Network (WSN) energy cost by transmitting only unexpected information and is so-named because "nobody expects the Spanish Inquisition!" SIP extends prior Dual Prediction Scheme (DPS) algorithms that model phenomena at both node and sink. SIP's key advancement is that it transmits a state vector estimate rather than individual readings. SIP can be tuned according to the desired estimate accuracy, with lower desired accuracy typically leading to fewer transmitted packets. In simulation with real data, less than 5% of the samples needed to be transmitted to provide the sink with an accurate estimate of the sensor value (within 0.5 °C, in the case of temperature). SIP also significantly outperforms prior DPS results when using the same data sets. In deployment on Telos motes, SIP shows similar performance to the simulations.