It took my entire life, up until last week, to realize that “The Alphabet Song” (the one that goes, “A, B, C, D, E, F, G...”) and “Twinkle Twinkle Little Star” have the same tune. Since then, I have asked several people whether they ever noticed this. No one had.
A few details and links: “The Alphabet Song,” originally from the 1830s, uses the same tune as “Twinkle Twinkle Star,” which in turn is a combination of the 1806 poem, “The Star,” by Jane Taylor and the 1761 French melody “Ah! vous dirai-je, Maman.”
If you want to see the various other ways the tune has been repurposed, including by Mozart in 1778, follow the links above.