Guys
When a baby boy is born, he has all the parts of his reproductive system in place, but it isn't until puberty that he is able to reproduce. A man's reproductive system is made up of the penis, scrotum, testicles, vas deferens, epididymis, seminal vesicles, and prostate gland. Some of these parts are visible, whereas others are hidden inside the body.

Penis
The penis is actually made of two parts: the shaft and the glans (pronounced: glanz). The shaft is the main part of the penis, and the glans is the tip (sometimes called the head). All boys are born with a foreskin, a fold of skin that covers the glans. Some boys have a circumcision (usually done within a few days after birth), which means that a doctor or a clergy member cuts away the foreskin. Whether a boy is circumcised can depend on the preference of the individual family, the part of the world he lives in, or his family's religion. Boys who have circumcised penises and those who don't are no different: all penises work and feel the same, regardless of whether the foreskin has been removed. At the end of the glans is a small slit or opening.

The inside of the penis is made of a spongy tissue that can expand and contract. When a guy is sexually aroused, or sometimes for no apparent reason, special tube-like passageways in the tissue fill up with blood and cause an erection. When this occurs, the penis becomes hard and straight and stands away from the body. Inside the penis is also the urethra (pronounced: you-ree-thruh). The urethra is also part of the urinary system. The urethra carries urine from the bladder, through the length of the penis, and out of the small opening in the glans.

Scrotum
The scrotum is a loose pouch of skin that hangs behind the penis. It is also sometimes called the scrotal sac. The scrotum's job is to hold and protect the testicles. The testicles make sperm, and to produce sperm the right way, their temperature must be lower than it is inside the body. The scrotum is designed to keep the testicles on the outside of the body and therefore at a lower temperature (at 92 or 93 degrees Fahrenheit [34 degrees Celsius], or about 6 degrees lower than normal body temperature).

The scrotum is so good at doing its job that it can even change size to maintain the right temperature. In cold weather, the scrotum shrinks and becomes tighter to hold in body heat. In warm weather, it becomes larger and more floppy to get rid of extra heat. This happens involuntarily - the brain and nervous system give the scrotum the cue, without a guy ever having to think about it.

Testicles
The testicles are two egg-shaped organs that are each about 2 inches (5 centimeters) in length in a grown man. They are sometimes also called testes. The testicles are contained in the scrotum. The scrotum protects the testicles and holds them outside of the body, where they can maintain a cooler temperature. One testicle almost always hangs down slightly lower than the other in the scrotum.

The testicles need to be at a temperature of 92 or 93 degrees Fahrenheit (about 34 degrees Celsius, or 6 degrees cooler than normal body temperature) so they can make sperm, which are very sensitive to warm temperatures. When a boy reaches puberty, special coiled tubes inside the testicles begin to make sperm cells. From this point on, the testicles continue producing sperm for the rest of a man's life, at the rate of hundreds of millions each day. Each sperm is extremely small: only 1/600 of an inch (0.05 millimeters) long.

The testicles are also responsible for making the hormone testosterone. Testosterone is a major part of puberty in guys, and as a guy makes his way through puberty, his testicles produce more and more of it. Testosterone is the hormone that causes guys to develop deeper voices, bigger muscles, and body and facial hair, among other things.