"Wear the Mask,"  the name of the app that Saatchi & Saatchi in New York created for metal band Slipknot, sounds like an ode to Spinal Tap's Smell the Glove. But it actually refers to a song off the mask-wearing group's first album. The app begins with a series of multiple-choice questions that allude to the band's history and ultimately determine which of Slipknot's nine members you're most like. (Do you prefer to throw up from partying or from working out too much?) From there, you can take a photo of yourself that splits into nine pieces and can be mixed with pieces of a band member's mask. The resulting personal masks can be posted on Slipknot's 12-million-fan-strong Facebook page.  The challenge for Saatchi was to create something visually and mentally stimulating without the benefit of the band's music, to which its label holds the rights, according to Brian Carley, the Saatchi executive creative director who steered the app's development. Carley, a musician himself who played guitar in a band called Penfold,  worked closely with Slipknot percussionist Shawn "Clown" Crahan on the project, which Saatchi landed through a connection with the band's management company. (It's not every day you can call your client "Clown" and get away with it.) The app, which is available in iPhone and Android versions, went on sale this week to promote the release of Slipknot's best-of album, Antennas to Hell.