U2 have made their current European tour a testing ground for new material. The best of the new live songs is "Mercy," cut during the sessions for 2004's How to Dismantle an Atomic Bomb but not on the album. The band has reworked "Mercy" for the stage, turning it into a big bear hug of a ballad that almost sounds like it could have gone on Joshua Tree. "You're going to kill me, and I want to die," Bono sings against the Edge's spacey arpeggios. "We were meant for each other." Other tracks that thrilled fans this summer include the intensely rocking "Return of the Stingray Guitar," the psychedelic "Glastonbury" and the starkly beautiful "Every Breaking Wave." The latter recalls "All I Want Is You," with Bono delivering dark poetry like "The waves know we're on the rocks/Drowning is no sin." U2 say they have four separate albums in the works; if so, this is a good start.

Photos: U2: The Rolling Stone Covers

“Youth Knows No Pain”

“Like on 'Youth Knows No Pain' — we are the ones that should demonstrate, because we can take it,” Likke Li said. “We can pierce ourselves, take Ecstasy, dance all night and still go to work at our McDonald's jobs.” Despite the hedonistic sentiment in the song, the Swedish singer also admitted in hindsight her youth had repercussions. “I remember when I was 18-19 and feeling that I know it all,” Li said. “I always feel that I know it all. But that song is about realizing you don’t, and reflecting, ‘Boy, if I only knew what would follow.’”