Correlation vs. Causality: The Cartoon Version

I don’t know why, but academic economists just love to use cartoons in their presentations. I would guess that one out of three academic seminars includes a cartoon, and if it is a plenary talk, a cartoon is virtually guaranteed.
I don’t have anything specifically against cartoons, but to the best of my recollection, I have never in my life included one in my presentation, or linked to one from this blog — until today.
My former student Hays Golden sent me the following cartoon from xkcd.com, and it is so appropriate given the classes I have taught this term, that I can’t control my economist urges. I have no choice but to post it here on the blog:

From xkcd.com