This is a good basis for open and honest communication about sex and relationships, growing up and going through puberty.
Children need to know about sex, pregnancy, contraception and safer sex before they start any sexual activity.
This is so they will know what to think about, such as safer sex and not doing anything they don't want to do.
Evidence shows that children whose parents talk about sex openly start having sex at a later stage and are more likely to use contraception. If they seem happy with your answer and don't ask a follow-up question, you've probably given them enough information. For example, if your three-year-old asks why she hasn't got a penis like her brother, you could tell her that boys have penises on the outside and girls have vaginas on the inside. You could answer by saying: "Babies grow in a woman's tummy, and when they're ready they come out into the world". If not, your child's follow-up question could be, "How does the baby get in there? They need to know that it's OK to talk about sex and relationships, and that you're happy to talk about it.
If they ask another question, you can tell them more. They'll learn this through your tone and manner when you talk about sex, so try to treat sex as a normal, everyday subject.
Girls need to know about starting periods before they're around 10 years old, and boys need to know about the changes they can expect before they're around 12.