"Who Knew" is a song by American recording artist and songwriter Pink, which was taken from her fourth studio album I'm Not Dead (2006). The song was written by Pink, Max Martin and Lukasz Gottwald, while it was produced by Max Martin and Dr. Luke. "Who Knew" was released as the second single off the album worldwide in 2006, while it was re-released to U.S. radio as the album's fourth single in the United States in 2007, after the success of "U + Ur Hand". Musically, "Who Knew" is a pop rock song, which has smaller elements of ballad and soft rock music. Lyrically, it is about losing a friend because of drugs. "Who Knew" received positive reviews from most music critics, who noted it as a strong song on the album and praised the lyrics, however some critics thought the song was too similar to singer Kelly Clarkson. The song was commercially success worldwide, where it peaked in the top spot in countries including Australia, Czech Republic, Germany, Hungary and component charts in the United States. The song is Pink's longest chart runner on the Hot 100, spending 36 weeks before dropping out.