During puberty, things happen inside the body, too. During puberty, most girls start having periods. Most boys start having wet dreams. And girls and boys begin having a lot more thoughts and feelings about sex. Sometimes they get really turned on.
Important changes happen inside the body by the time wet dreams and periods begin.
These changes mean that the reproductive system is maturing. As boys mature, they begin producing sperm. Sperm are the male reproductive cells. As girls mature, the eggs in their ovaries begin to ripen. Eggs are the female reproductive cells.