In Friends, the male babysitter Sandy is seen playing a song on the flute (1:50).
The same song is played by Sheldon Cooper, also on the flute (58:46).
After some digging, I found the song to be Greensleeves.
This is an english folk song.
What is the relevance of this song to the shows?
Is TBBT referencing Friends, or are both shows referencing this tune? Is this a famous flute song (or often taught for flute learners)?