Why Bruno Mars and Lady Gaga’s “Die With a Smile” is Billboard’s No. 1 song.

Throughout chart history, there’s a special breed of duet by two recording artists so illustrious I can abbreviate each name with a mononym, and you know who I’m talking about and probably even the song. “Die With a Smile,” the first-ever collaboration between Peter Gene Hernandez, aka Bruno Mars, and Stefani Germanotta, aka Lady Gaga, takes over the Hot 100 as the annual onslaught of Christmas music recedes. And these two have been at the pop game long enough that it’s hard to say there’s any one “Bruno sound” or “Gaga sound”; you’re not going to find any of her art-damaged dance pop à la “Poker Face” or “Bad Romance” in here, nor does he traffic in the ’80s style-jacking of “Locked Out of Heaven” or “Uptown Funk!” By 21st-century songwriting standards—modern pop hits can have a dozen or more collaborators—the two kept their circle relatively tight. Mars leaned on his frequent collaborators Dernst “D’Mile” Emile II and James Fauntleroy, and Gaga brought in journeyman rock producer Andrew Watt, who was working on her forthcoming album. It was supposed to be the new Gaga album’s first single but has effectively been replaced by “Die with a Smile,” a Bruno Mars side project that has taken on outsize importance in Gaga’s comeback.