The barrage of media attention surrounding the HPV vaccine for young girls has sparked parental concerns. Among them: What do kids understand about sex at what age—and when and how do you talk to them about it? Be prepared. Your preschooler may soon hit you with "Where did I come from?" and "What are you and Daddy doing?"—if she hasn't already.

Parents often think 3- and 4-year-olds are too young to talk about sexuality, but this may be more about their own anxiety than their child's, says Justin Richardson, MD, an assistant professor of psychiatry at Columbia and Cornell universities in New York City and a coauthor of Everything You Never Wanted Your Kids to Know About Sex (but were afraid they'd ask). "There is no right age to begin the talk," he adds, "but if you start sooner rather than later, you'll feel more comfortable about it as your child grows."So even before the questions come, it's appropriate to tell your preschooler, "That's a penis and that's a vagina." It will help lay the groundwork for later conversations about conception and puberty—and make it easier for both of you, says Dr. Richardson, who strongly recommends using the real words for body parts along with the cute words.

If your child walks in on you and your partner having sex and asks about it, cover up and have her step out of the room with you, suggests Dr. Richardson. Then, in a casual but sensitive way, say something like "We were doing something adults do to show love, called having sex." Be sure she isn't led to feel she's done something wrong, but let her know she needs to knock first when your door is shut.

For the "What is sex?" query, say something like "That's a really good question. Why don't you tell me what you've heard." Then respond appropriately. Even at this age you can tell her the whole story. "Mommy and Daddy get really close, and an egg from Mommy's body and sperm from Daddy's body come together to form a baby." You can talk about the penis going into the vagina—"it won't disturb your child," Dr. Richardson says. "But you don't have to if you're uncomfortable." As for where babies come from: "You grew in a special place in Mommy's body called a uterus. When you were ready to be born, you came out from my vagina." If your child says "Eeew," no problem. She's learning to get comfortable with the topic—as are you.