It’s not like Willow Smith needs another hit on the scale of “Whip My Hair.” Not only because what kind of 10-year-old needs another viral single, but also because Willow’s got her parents and she’s going to get to make Annie no matter what. But still, here comes her next single, “21st Century Girl.” It’s less rock-ish than “Hair,” and Smith’s voice sounds more meddled with. But it’s got a basic, highly sticky underlying melody you will be humming instantly, and like “Whip My Hair” it has an anthemic, seemingly age-appropriate message. (How Smith’s management made a song called “Whip My Hair” sound and seem like it had everything to do with palatable rebellion and nothing to do with sex is a high-order magic trick.) There is one line, “I’m the type of chick who likes to rock the beat,” that Smith delivers, uncomfortably, with a little moan in her voice. But then the chorus kicks in — “21st century girl / I’ll do what I like” — and well, you can whip your hair back and forth to it.