From: jonathanthio@yahoo.com.sg   
      
   If 2 variables correlate we can't imply causality. It could be that   
   the 1st variable cause the other. It could be the other way around. It   
   could be that they both correlates with a third variable.   
      
   I've heard that if we know 3 variables, then we can imply causality.   
   How in the earth the 3rd variable help us that?   
      
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