Impressive statistics are thrown at us every day - the cost of health care; the size of an earthquake; the distance to the nearest star; the number of giraffes in the world. We know all these numbers are important - some more than others - and it's vaguely unsettling when we don't really have a clear sense of how remarkable or how ordinary they are. How do we work out what these figures actually mean? Are they significant, should we be worried,...