Humming a tune

I always sing the same song to Evi when I put her to bed ("Sleep, Baby, Sleep") I often wonder whether this is appreciated because she is often squirming and trying to get out of my arms and only relaxes when I finally lay her down in the crib (maybe this is her version of, "Well, if you're going to put me to bed, Mom, let's just get it over with!" Anyway, I still always sing it regardless. Tonight after we had been looking at books I told her that it was time for her song. She promptly put the book on the nightstand and started singing the first three notes of the song herself! She wasn't matching my pitch exactly, but it was actually close enough that it completely astonished me!