Here's another one that falls in the "How did they memorize this?" category. Actually, it's only true of the lyrics, which you'd imagine were sung off of a lyric sheet. It would certainly be remarkable to learn that they did this one live!
What's difficult to remember? The verse in the song is 32 bars long. And there's three of them.
Seventy words in the first verse, seventy-one in the second, and sixty-seven in the third, all different. If she was memorizing this stuff, then Martha Reeves was ready for Shakespeare next.