We’re not all statisticians but we are all, to some extent, programmed to reason statistically. In a bid to make sense of the world around us, we compare, contrast, look for patterns and are drawn to a statistical technique called correlation, a way of measuring the extent to which a change in one measurable thing – a ‘variable’ – is associated with the change in another measurable thing.
Indeed, you can calculate the correlation between pretty much any two things which can be quantified, counted and measured. But Statistics doesn’t operate as a set of techniques, its value is in providing insight into a problem, so if you are going to calculate correlations it makes sense for there to be some reason for doing it i.e. because you want to take action of some kind.