Did you know that when ice cream sales rise, so do property crime rates?
What do you think causes the two events to occur together?
The explanation is that property crime peaks in the summer because of warmer weather, more people on vacations (leaving their homes vacant), teenagers out of school, and so on.
It just happens that ice cream sales also peak in those months because of the weather.
It is a case of a third variable causing both to occur.
Or what if there were a positive correlation between sales of cigarette lighters and the incidence of cancer?
The suspect might well turn out to be the omitted variable (the so–called “smoking gun”): the cigarette.
Or what if research revealed that parents who bought parenting books were “better” parents?
Does that prove the books work?
Or is it possible that people who would buy books on parenting tend to be “better” parents?
That is, it might be about the parents, not the book.
Causality is tricky stuff.
Be careful.