Once you find out what she thinks sex is, begin by telling her that sex can mean several things. Sex has to do with the body parts a person is born with, how people feel about others and a way for adult couples to share pleasure and create a baby if they want to start a family. One's sex at birth is either female or male. This is determined by the sexual organs. Girls have a vulva with a vagina, clitoris, uterus and ovaries.
Boys have a penis and testicles.
This is what makes boys and girls different from one another. Sometimes a person's sex is referred to as his or her gender. Sexual attraction and sexual identity may be a bit trickier to explain to your daughter. You might want to say that as she gets older and matures she will begin to have special feelings for other people. They are usually strong feelings that she'll probably have for a few people in her lifetime.