Why does correlation not equal causation?

In this blog post I will explain why it makes sense to think about causal explanations when we see correlation, why correlation does not always imply causation, and which alternative causal models to consider when you are trying to figure out why two variables in your data are correlated. The blog post is intended as a non-technical introduction. I use only words and pictures to explain all concepts and logic, and I use a hypothetical example from health psychology to illustrate. The post should be relevant to anyone who is interested in using data to understand causal mechanisms.