The human penis is a male body part found on the outside of the body. It is used for urination and for sexual reproduction. The main sexual function of the penis is to be inserted into a female's vagina and deliver semen which may cause pregnancy. This activity is called sexual intercourse.

There are many slang words for penis. They include dick, cock, and schlong. A euphemism for penis is male member.

Structure 

The human penis exits the abdomen above the scrotum and hangs freely outside the body. The main visible part is called the shaft.
The human penis has three parts.
Body of the penis: Main visible part of the penis. The urethra runs underside of the penis.
Root of the penis: Invisible part of the penis. It reaches near the anus and contains the bulb of penis and the crus of penis. The crus of penis is attached to the pubic bone.
Skin of the penis: The shaft is covered by the skin. The skin is not attached tightly to the inner tissue. So it can move freely on the shaft.

The human penis is made up of three rod-shaped tissues. It has two corpora cavernosa (special pieces of muscle) next to each other and a corpus spongiosum (spongy muscle) between them.
The end of the corpus spongiosum forms the glans penis. The glans penis is covered by the foreskin in uncircumcised males. The rounded base of the glans is called the corona. The area on the bottom of the penis, where the foreskin is attached, is known as the frenulum of prepuce. 

The urethra, the tube where urine and semen travel through, runs down the corpus spongiosum (spongy tissue), and opens at the tip of the penis. The opening is called urinary meatus. Sperm are made in the testes (ball-like organs) and stored in the epididymis (layer of tissue) around the testes. During ejaculation, sperm are pushed up the vas deferens. Fluids are added by the seminal vesicles, the prostate gland and the bulbourethral glands to make semen.

In sexual reproduction

Erection 

A penis can become erect if a  male is sexually aroused (or sometimes during sleep, even though there is no sexual stimulation). In an erection, the penis fills with blood. The blood makes the penis become longer, thicker and harder. Veins taking blood away from the penis get smaller, so less blood is taken through. Arteries bringing blood to the penis get wider, bringing more blood to the penis.

Ejaculation 

Ejaculation is when semen is pushed from the penis. It usually happens during an orgasm. Males can ejaculate during sexual intercourse or by masturbation.

In reproductive sexual intercourse between a male and female, the erect penis is inserted into the vagina and moved in and out. The vagina places pressure on the penis, which can cause the male to have an orgasm and ejaculate into the vagina, causing insemination.   

Other forms of sex, like anal and oral sex, can also cause stimulation, and therefore orgasm and ejaculation. In masturbation, a male can stimulate the penis and other sensitive areas of the body, such as the scrotum, by rubbing the genital area. Ejaculation may also happen during sleep (called a 'wet dream'). A male must have started puberty before he can ejaculate. During orgasm, muscles push semen from the penis. Semen moves through the urethra and comes out of the hole at the tip of the penis.

Penis size in humans 

The length and thickness of the penis is different for different people. The size of a soft penis (not erect) is much smaller than when it is erect. Some penises grow more when they get hard than other penises. In most cases, whether a penis is big or small, it can still be used for sex. It averages out to be around 3 to 6 inches long when not erect. The average size of an erect human penis is between 13 – 16 cm (5.1– 6 in). The average circumference of a penis is 12.3 cm (4.85 in) when fully erect. The penis grows bigger during puberty. At the start of puberty, the average length of the penis is . The penis reaches adult size about 5 years later. A study done in 1996 found the mean length of an adult man's penis is  when it is not erect. The average length of an erect penis is about .

Circumcision 
The foreskin is a fold of skin that covers the end of the penis. Cutting off the foreskin is called circumcision. The foreskin is connected to the head of the penis. During circumcision, the foreskin is removed from the penis. Circumcision is usually performed on infant males for medical, religious or cultural reasons like in the USA, Philippines and South Korea.
In Europe only Jews and Muslim men are circumcised of religious reason. Uncommon by christian europe male. 

Some males have the foreskin cut off when they are adults because they have problems with their foreskin. Some males have the foreskin cut off because they want to change how their penis looks.  

In common speech, you can say that someone is cut or uncut, meaning circumcised or not circumcised . 

In some religions, babies and young boys have their foreskin cut off. This is expected in Islam and Judaism. It is not required according to Christianity. In Judaism, infant males are required to have their foreskin removed as a sign of the covenant (ancient promise) made with God.

Skin color
A young boy's penis skin is the same color as his other skin. Around age 12-15, a boy's penis skin gets slightly darker. This is normal because of puberty. This is caused by sex hormones (natural chemicals), especially testosterone. Testosterone makes more melanin. Melanin is the amino acid that causes skin color. (Sunlight also makes more melanin so skin gets a sun tan.)
Usually a teenager or man's penis side skin is slightly darker than other skin on his body. The tip (glans) of the penis is usually pink or red color even if the penis side skin is dark. If the man is not circumcised, the tip is usually covered by the foreskin. The foreskin is about the same color as the skin on the side of the penis.

If a man's penis skin changes color and he doesn't know why, he should go to a doctor. He could have an infection, diabetes or cancer. A sunburn on the penis can happen quickly.  If a man is outside without clothes, he will get a sun tan. That is normal. Too much sun causes a sunburn. Doctors say to use sunscreen (sun cream) before and after nude swimming. A sunburn can happen in a few minutes on a very sunny day.

Culture
Since the oldest cultures the penis has assumed a strong importance as a symbol of fertility used also in religious ceremonies to propitiate the reproductive capacity of men and beasts. For example, in Roman paganism the god Priapus is characterized by a huge penis.

Related pages 
 Phallus
 Scrotum
 Testicles
 Vagina
 Fellatio

References

Other websites 

 Kinsey Institute on the penis  

Anatomy of the male reproductive system