"Pop Goes the Weasel" is an Earth nursery rhyme, often set to a melody.
Data attempted to whistle the melody on the holodeck during the Enterprise-D's mission to Farpoint Station, shortly before meeting Will Riker for the first time. When Riker found Data, he finished the last few notes, causing Data to marvel at the ease at which humans could do so. (TNG: "Encounter at Farpoint")
Data still had difficulties with the tune by 2367, when he met his "father," Noonien Soong. (TNG: "Brothers")
Shortly after Data sacrificed himself to destroy the Reman warbird Scimitar and save the Enterprise-E, the senior staff gathered in Captain Jean-Luc Picard's quarters for a small memorial. Riker offered the story of his first encounter with Data, but was unable to recall what Data was attempting to whistle. (Star Trek: Nemesis)