This is what she does. She takes a pretty good song, waits until you are popping along to it, then a little longer, until it feels repetitive and you start to see through to its flaws, and then boom, she comes in and makes it a completely different song—a better song. She is the best part even of great songs; her featured verse on Kanye West's "Monster" is the best of several, including ones by Jay Z and Rick Ross. She did that song because she was asked and because "Kanye's a genius." She did "Bang Bang" because she "knew it would be big."