My favorite example is to do the following: 1. Get data on all the fires in San Francisco for the last ten years. 2. Correlate the number of fire engines at each fire and the damages in dollars at each fire. Note the significant relationship between number of fire engines and the amount of damage. Conclude that fire engines cause the damage. The reason that I like this example is that the conclusion is so absurd. Anyone will quickly recognize that both variables result from and are correlated with the overall size of the fire. However, many spurious correlations do not seem absurd and some seem compelling.