MAKING NUMERICAL SENSE, Part 2 

Remember how we used common sense to make "numerical sense" of certain 
ways of the universe?  It's equally important, and perhaps even more 
common, to make numerical sense when we talk about the matter in the 
universe.  That's because the things we ordinarily encounter in our 
lives are limited in their range of sizes, masses, and composition.  
When we start doing astronomical calculations - counting atoms, 
planets, stars, galaxies, and much more - we start running into 
unfamiliar territory in a hurry.

In these cases, one of the most useful things we can do is to break 
complex problems and large numbers down into their component parts.  
For example: how high is a stack of a million sheets of paper?  Most 
of us never deal with millions, but we do generally encounter reams of 
paper - such a stack of 500 sheets is about 5 centimeters (2 inches) 
thick.  So a million sheets is 2000 reams, at 5 cm per ream - that's 
10,000 cm, or 100 m, or about the length of a football field.  One or 
two intermediate steps is all we need to get a grasp of what might 
otherwise seem an intimidating or intractible number.