Once upon a time, there was a pair of Testicles . They were held in a special sac called the scrotum. This sac could hold the testicles close to the body to keep them warm, or let them hang away from the body to keep them cool. The testicles made special reproductive cells called sperm. Once these cells were made, they would wait to be released from the testicles. Sometimes, they would wait so long that they dissolved. Other times, they would be released from the testicles, make a journey through the male reproductive system, and leave the male’s body. When the sperm leave the penis, it is called ejaculation. On the day in question, it just so happened that the sperm got to be released from the body. First, the penis became larger, longer and firmer until it stuck out from the body. (When the penis gets this way it is called an erction. Erections are a normal process of growing up, and can happen for physical reasons or because of sexual arousal.) Then the sperm traveled up the vas deferens. Along the way, it mixed with prostatic fluid from the prostate gland and seminal fluid, which was made in the seminal vesicle. Once these fluids mixed, they decided to call themselves semen. Together they traveled from the vas deferens into a tube called the urethra. (There are two branches to the urethra, one from the bladder and the other from the vas deferens. When the penis is ready to release semen, a valve blocks off the branch to the bladder so urine cannot escape. Only one of the two fluids can pass through the urethra at a time.) By this time, the semen was almost at the end of the journey. After traveling through the vas deferens and the urethra, the semen was released from the penis in a process called ejaculation. The erection went away, and the penis became smaller and softer.