Britney Spears has been speared -- or, rather, pierced. Her belly button, that is. But that's as far as the body work goes, she insists in the Feb. 14 issue of PEOPLE. "Like I'm really going to get breast implants at 17?" the sexy pop sensation, now 18, tells the magazine. "I think it's fine. But 16, 17, 18 is too young for any surgery." She also claims that she and 'N Sync's Justin Timberlake are "just friends," and though she and one of her fans, Britain's Prince William, have exchanged e-mails, they won't be having a Valentine's Day rendezvous, as has been widely speculated by the British press. "He's very cute and so sweet," Britney says of the monarch, 17. "But people blow things out of proportion." As for her current companion, Britney reveals she's a girl . . . Mitzi, a Yorkshire terrier puppy she received from her parents for Christmas. "It's so good," she says of having a pet, "because when I go home I have somebody who misses me."