dealing with callbacks is a 2-faced endeavor.  On one side are functions written to handle callbacks, on the other side is using these functions

functions which execute callbacks:
	a normal function
	with an extra argument which is a function
	in place of a return statement, they execute the callback function

using functions which execute callbacks:
	1) decide the two things you want to happen and in what order
	2) the first one will be the function that takes callbacks
	3) the second will be the function passed into the callback

why callbacks? to guarentee the order of things happening.
you've heard node is asynchronous:
	lines of code are initiated in order
	but executed at their leisure
so: 
	console.log the result after doing the math
	rerender home page AFTER new data is added  
  
[a good article](http://blog.gvm-it.eu/post/22040726249/callback-conventions-in-nodejs-how-and-why)