The Ice Cream Murder Mystery
Observed pattern:
As ice cream sales increase → drowning deaths increase
As ice cream sales decrease → drowning deaths decrease
Perfect correlation!
Conclusion 1 (wrong): Ice cream causes drowning! Ban ice cream!
Conclusion 2 (wrong): Drowning makes people buy ice cream!
Reality: Summer weather causes BOTH. Hot weather = more swimming (drownings) AND more ice cream sales.
This hidden factor is called a confounding variable.
This is why correlation ≠ causation.