IU debuted in 2008 at the age of 15, and soon became known for her clear-voiced take on energetic bubblegum pop songs and hushed, solemn ballads. But her breakthrough single, “Good Day” (with its famous, lung-busting three high notes at the climax) emphasized the strength hidden in her delicate-sounding voice. As her career has progressed, she's explored other styles: later songs like “The Red Shoes” reveal a taste for the big band jazz sounds of the '30s and '40s, as well as more refined acoustic café pop.