Tis the season to ... watch our weight. Researchers at the University of Colorado thought, "If we watch our weight, maybe we should watch Earth's weight too". After all where does all the Christmas pudding go that isn't eaten? Seriously, scientists with the Gravity Recovery and Climate Experiment (GRACE), use two satellites flying about 220 kilometres apart to continously monitor the distance between them to within a hundredth of a millimetre. Using this measurement, it's an "easy exercise for the student" to calculate the local gravity field and check for any fluctuations in water distibution or density. The scientists clain that GRACE should be able to measure a 4-millimetre change in water height across the 32 million square kilometres of the Mississippi river basin. See the article in December 10th's Nature.