Behind the penis is the scrotum, which contains the testes, where the sperm and the male sex hormone (testosterone) are made. The sperm leaves the testicles and goes into a tube called the epididymis, and then the vas deferens, where it is stored.
When a man ejaculates, the sperm mixes with fluid from the prostate and seminal vesicles to form semen. During sex the semen travels through the urethra and into the woman’s body through her vagina.
From the vagina it travels through the cervix, into the uterus and then up into her fallopian tubes. If one of the sperm meets an egg then fertilization happens and an embryo is formed, which becomes a baby.