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. Important changes are happening inside the body. The reproductive system is maturing.
Boys begin producing sperm - the male reproductive cells.
Girls ovaries begin to ripen eggs - the female reproductive cells. Pregnancy can happen if only one sperm joins with an egg. The sperm can reach the egg if a boy or man puts his penis in or near the vagina. 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.