Sex is the way most babies are made. It's a special, grown up way of being close to someone. When people have sex or make love, it feels good to their bodies.
Sometimes grown-ups have sex because they want to make a baby. Sometimes people also have sex when they don't want to make a baby, because they like the way it feels.
Because a baby can be made from sex, it's really important for people to be careful and make smart decisions if they want to have sex but don't want a baby. Sex is definitely not for kids because kids' bodies, hearts, and minds aren't ready for this kind of closeness. Kids need to wait a long time before making this kind of very grown up decision. You can talk about it a lot with the trusted grown-ups in your life as you get older. This will help you be really ready for this big decision. Make sure to tell a trusted grown-up right away if anyone ever does something that makes you feel uncomfortable or touches your private areas.
(by Mya Kagan)
The word "sex" also describes whether a human or an animal is male or female. For example, if you get a new puppy, someone might ask "What is the sex of your dog?" This person would be asking is the dog male or female.