Bluematter.

Geddit? As an economist talking to non-economists, this has been the single most difficult fact to explain.

This is from Timothy Taylor's 'Principles of Economics' lecture series on video. Click the link and check the list - he is a big success on the web, and - though his speaking style is a tad bit overstated - I really like this guy!

If you clicked on the link and are perplexed, this may help explain things.