What is the difference between correlation and causation, and why is confusing them a classic thinking error?

Correlation is a statistical pattern (two things move together); causation is a cause-and-effect link — and you cannot reliably infer the second from the first.

Two variables can rise and fall together for reasons that have nothing to do with one causing the other:

• Correlation — a statistical association: when A is high, B tends to be high.
• Causation — A actually produces B.

The classic illustration: ice-cream sales and sunburns climb together, but neither causes the other — a third factor, hot sunny weather, drives both. Treating a mere correlation as proof of cause is one of the most common reasoning errors, and it powers a lot of bad health, political and economic claims. The discipline is to ask: could a hidden common cause, or pure coincidence, explain this pattern instead?