A Sword in a Field -- Correlation is Not Causation

Consider the undeniable correlation between shoe size and reading skills. If you were to conduct a study across the country, you'd find that the larger shoe sizes mean better reading skills, but does that necessarily mean that shoe size influences reading abilities in any way? No, even though there is a correlation between shoe size and reading skills, the obvious reason is that young children who cannot yet read or who are just learning to read have smaller feet.

By adding another independent variable to our study... age... we'd find out that it wasn't shoe size that was making for better readers, it was age.''