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

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?