Why you can remember every word of a song from 25 years ago – but not why you walked into the room

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While driving recently, a long-forgotten song came on the radio. I found myself singing along; not only did I know all the lyrics to a song I hadn’t heard in 25 years or more, but I also managed to rap along. How is it that I could give this rendition, but often cannot remember what I came into the room for?

It is tempting to treat these moments as evidence of cognitive decline. A quiet, creeping sense that something is slipping. But the contrast between flawlessly...