The Earth nursery tune that Data is struggling to whistle when William Riker first meets him on the holodeck in Encounter at Farpoint, later used by Riker as a key to Data's memory.
Pop Goes the Weasel is an old Earth children's tune. In Encounter at Farpoint (2364), the newly arrived Commander Riker goes looking for Data and finds him in a holodeck woodland, trying without success to whistle it. Riker supplies the last few notes, and the two have their first conversation about the android's wish to be human.
The scene is one of the first statements of Data's character: a machine of enormous ability, defeated by something any child can do and determined to master it all the same.
In the Star Trek: Picard episode The Bounty, set decades later, Riker hears a string of musical notes played by the security system of Daystrom Station, recognises the tune and whistles the ending, guessing correctly that part of Data lies behind the system.