Correlation, not causation

Sometimes correlation exists between two variables but this does not automatically mean there is causation.

An example of this is that the sale of ice-creams is highest when the number of accidents in swimming pools is highest. To claim that ice- cream sales cause accidents in pools, or vice versa, would be false. In reality, both factors may have been caused by a heat wave. The heat wave is a hidden or an unseen variable, also known as a confounding variable.