I Know What You Did Last Summer

Mendes started writing this love duet with Fifth Harmony's Camila Cabello when the pair met up backstage at one of Taylor Swift's 1989 World Tour shows. In the summer of 2014, Mendes and Cabello were opening acts on Austin Mahone's tour, but they didn't spend much time together. The next summer, they got together at the Swift show - Mendes was an opening act and Cabello went to see it with Ally Brooke of Fifth Harmony. Mendes pulled out his guitar and they started playing around, which led to this song. "Working with Camila was amazing," Shawn told MTV News. "We wrote this song backstage so spontaneously."

In a behind-the-scenes look on the making of the song, Cabello admitted that the song was an emotional one. "It's basically a conversation between two people in a relationship where it's dying but nobody wants to admit that it's dying," she explained. Elaborating on the specific inspiration in Rolling Stone, Cabello said that she had been talking to a guy she liked, but found out from one of her girlfriends that the same guy was flirting with her as well. "It's about finding out that somebody had been cheating on you, and the song is that moment right before you confront that person where you both know what's next, but it's killing the both of you," she said.

I Know What You Did Last Summer is the name of a classic horror movie from 1997 starring Jennifer Love Hewitt. Cabello and Mendes claim the song has nothing to do with the film, and the title is pure coincidence. They didn't realize it was a movie title until after they started recording the song: Cabello was born in 1997 and Mendes was born in 1998. In the film, a group of teenagers accidentally kill a guy; in the song, the transgression is cheating.