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.
Pregnancy can happen if only one sperm joins with an egg. So, a girl or woman can get pregnant if a boy or man puts his penis in or near her vagina. The sperm from his penis can swim up her vagina to join with her egg. Often our reproductive systems grow up faster than we do. That's why girls can get pregnant and boys can cause pregnancy before they finish growing up.