[hallway_monitor]

This is an amazing explanation and I am going to keep it on hand for future use. In the first sentence causal is typoed as casual

[pepa65]

And "However, the two variables would be correlated if the causal arrow were reversed" is missing "also", almost suggesting that the article gets it wrong and the two variables are not correlated because of the placement of the causal arrow...

[tmoertel]

Thanks for your kind words! And thanks for reporting the typo (now fixed).