Click to listen to Britney Spears' "Till the World Ends"

You want a party song, call a partier. Written by Ke$ha (with Max Martin and Dr. Luke), the second single from Femme Fatale is ginormously pumping uber-Euro uber-disco: sky-sucking synth streaks, a beat that sounds like blimps fucking and a thousand shirtless drunken sailors chanting along on the chorus. Her gauzy vocals kind of drown in the tsunami track. But so would Pavarotti.

Britney Spears: A Life in Photos

“The A Team”

This debut track from the then-20-year-old British singer-songwriter has a dark story behind it. Sheeran says he culls songwriting inspiration from "viewing other people's situations," which, for the heroine in "The A Team," involves drug addiction and prostitution that began as a teen. Sheeran paints the woman's trials with haunting imagery such as "But lately her face seems/Slowly sinking, wasting/Crumbling like pastries." "I did a gig at a homeless shelter, [and the song] is about one of the women there. It's her story," he said.