Nellie McKay's keen wit sharpens the singer-composer's performances in concert halls and on her five albums, most recently Home Sweet Mobile Home. Her pop-cabaret songs also have been heard on film and TV sound tracks, including Grey's Anatomy and Weeds. Veg since age 8, McKay is an outspoken advocate for animal rights. While gearing up to tour her musical revue I Want to Live! she spoke with VT from her Manhattan home. Q You've always been vocal about issues that matter to you. Do artists have an obligation to use their fameto educate and enlighten?A I don't know if it's an obligation, but it gets awfully boring otherwise. When you're not getting a reaction, that's when you should worry. You want people to care. I like the idea that music can get into people's minds, hearts, and souls. Then, maybe slowly, a lyric will cause them to start rethinking their lives and choices. Q How would you advise dealing with people who strongly disagree with you, say, about eating meat?A I generally think humor is the best weapon. Q What inspired "Unknown Reggae," in which you sing about a hamburger being someone's mother?A I was sitting on a beach, eating a veggie burger, thinking about life. As for the bridge of the song, I was walking in Central Park with my dog. Sometimes, from the sky, a little melody comes, and you have to write it down with a pink pen on the back of a receipt.