A popular-media take on a subtle problem in sampling. I found the graph quite illustrative.
http://www.theatlantic.com/business/archive/2012/05/when-correlation-is-not-causation-but-something-much-more-screwy/256918/
Incidentally, Pearl's original explanation in Chapter 1 of Causality is here; the whole first edition of the book is available online here.
A popular-media take on a subtle problem in sampling. I found the graph quite illustrative.
http://www.theatlantic.com/business/archive/2012/05/when-correlation-is-not-causation-but-something-much-more-screwy/256918/