We have realized that Vivian just cannot get the hang of singing songs. I know she knows the words and the tune, but she just can't figure out how to make the sing-song sound with her voice. So recently we have been singing songs with her and leaving out words so that she can fill in the blank. Hopefully it will help her learn faster. For example, we start the ABCs and leave out letters for her to say. ABC __ EF __ HIJ __ LM...
Anyway, the other night I was doing this with Twinkle, Twinkle Little Star. I stopped singing after "Little" so that she could sing "star". But she said, "Bug". I laughed at her and started over, and she said bug again! She insisted that it was bug. I know she knows the song...we sing it a lot to Kate, and it is on one of her DVDs. I was so confused.
Finally I remembered that one of her Elmo books has a version of the song that goes:

Twinkle, twinkle little bug
Elmo reaches for a hug
But up above the world so high
You cannot reach a firefly
Twinkle, twinkle little bug
Now Zoe's ready for that hug.

So we sang the song with those words and she knew it :) I guess Elmo even trumps classic lullabies.