2. I have noticed a strong positive correlation in my own life: when I do more laundry, I’m in a better mood. I first noticed this correlation in college, when I tended to feel a bit more upbeat during weeks when I found myself in the laundry room more than once and a bit more down on weeks when the pile of dirty laundry didn’t grow so fast. What’s up with that? Could it be that washing, drying, and folding caused my good mood? Or that my good mood caused me to do more laundry?

Neither of those made much sense. Eventually, I settled on a more likely interpretation: there was a third factor—exercising—that caused both. In weeks when I ran or played basketball more often, the exercise improved my mood, and it increased the amount of sweaty clothes in the hamper, which meant I needed to do laundry more often.

How about you? Have you ever noticed two things correlating in your own life—increasing together, decreasing together, or one increasing when the other decreased in a predictable way—but without a causal relationship between them? If so, what were they, and how do you explain their correlation?