Pop Goes the Weasel

Pop Goes the Weasel is a human children's song.

In 2364, when William Riker first met Data, the android was in the midst of an attempt to whistle the tune. After not being able to perform the last note correctly, Riker whistled it for him. 

Fifteen years later, Riker reminisced about that incident after Data's death protecting the USS Enterprise crew from the Scimitar's Thalaron radiation weapon. At the time, Riker could not remember which song Data has been attempting to perform.