Many celebrities make an effort to stay out of the spotlight, but there's only one who's made hiding her face from the public eye an actual part of her public persona: Despite having a number of smash hits under her belt, Sia is able to maintain relative anonymity in the real world by concealing her face behind a curtain of blonde hair during interviews and whenever she's onstage. Naturally, many of the singer's fans were surprised when she announced last week that she'd be lending (some of) her face for a good cause, as the latest star to sign on for a lip-colour collaboration with MAC as part of the brand's Viva Glam initiative.