I sing the same 2 songs to DS15mo every night. One of the songs has 4 parts with the same rhythm, and the second part has different lyric from the rest of them.
Something similar to this:
Part 1 => Lyric A
Part 2 => Lyric B
Part 3 => Lyric A
Part 4 => Lyric A
After I sing the 2 songs, I usually hum the 2 songs over and over again until he falls asleep.
Yesterday night, right after I finished humming the first part of that song, DS started to sign the lyric in the second part of the song. I did it a couple more times, and he only signed in the second part of the song. I am truly amused.
I might be overanalyzing. On one hand, I think DS might be more advanced. On the other hand, he is 15-month-old and can only say 2 words consistently.
