A recent story about noticing my confusion: I noticed that the 2-pound bag of coffee beans I had just bought was also listed as 908 grams. That sounded wrong, since I "knew" a kilo was about 2.5 pounds. I thought the manufacturer might be rounding the English weight down since the gram measure was more precise. But then when's the last time you heard of a producer rounding the weight down?

A quick trip to Google and damned if a kilo isn't 2.2 pounds, not 2.5. I've been misestimating that for years!

Of course, this raises the question of how I got 2.5 stuck in my head. I suspect it's a case of anchoring and similarity bias. I.e. there are 2.54 centimeters in an inch, and my brain learned the pattern that to convert metric to English units, the factor is 2.5. I probably started using that number without conscious attention or thought.