Adele dropped '23' -- and you didn't even know it!

While fans continue to buy up every copy of 25, her first album in four years since the release of 21, they may be surprised to learn that Adele had already kinda, sorta dropped 23.

During her concert special at Radio City Music Hall in New York City, which will air on Monday, Dec. 14 on NBC, the singer revealed that "Skyfall" actually filled the void between albums.

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"I was 23 at the time," Adele says, adding that since Skyfall was the 23rd James Bond film, it made sense for her to write and record the titular theme song that earned her an Oscar for Best Original Song.

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The now 27-year-old also reveals she was pregnant with her son, Angelo, when she recorded the track -- changing the register in her voice. "My voice was really low," she says, admitting it's hard for her to perform the song live. Her son was later born on Oct. 19, 2012, just two weeks after the song was officially released.

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In fact, the special was only the third time she ever sang "Skyfall" live following a performance at the 85th Academy Awards in 2013 ("That was a laugh," she jokes about the ceremony) and another during a BBC concert special that aired this past November.

The NBC special also features performances of several new songs, including the mega-hit, "Hello," and her latest single, "When We Were Young," which she says is her favorite that she's ever written.

Adele: Live in New York City airs Monday, Dec. 14 at 10 p.m. on NBC.