The best response to that question I've ever heard, to date, courtesy of John Green during his introduction of Crash Course World History #1:
(The awesomeness of answering is found in the first 55 seconds, though the rest of the episode is also pretty superb.)
by John Green, in Crash Course World History #1
via Jonathan Byrne (@jbteacherman)
Tags: Crash Course, education, John Green, testing