One thing I have always questioned is this: Teachers always tell us that a final answer should never have any more significant figures than the least number of sig figs on a measured value used. But something doesn't add up here..... Suppose you knew the length of a metal rod to be 1.000000 meters (you knew the value up to 6 decimal places; you know it really accurately.) And you know the length of another metal rod to be 1 micro-meter (you know it extremely accurately too, but there is only one sig fig because a micrometer is much smaller than a meter) Now, if you were to add the lengths, you would obtain 1.000001 meters, but because of the teachers' rule, you leave this as 1 meter. How is it possible that you know the value of a rod's length so accurately, and add on ~1 micrometer (almost no difference), and now you only know the final length to only 1 decimal?