The scrotum is the sack of skin that hangs below the penis. Inside are the testicles (testes), which are two small structures more commonly called "balls."

Inside the testes, sperm cells are made and stored. These are the cells that come out of the penis during ejaculation, or when a guy cums. If a man and woman have vaginal sex and he ejaculates on or in her vagina, the sperm can swim through a female's body searching for an egg to fertilize. If they connect with one, they can start a pregnancy.

The sex hormone, testosterone, is also produced in the testicles. Testosterone helps gives men their special traits – deeper voices, more hair on the body and an enhanced ability to build muscles.

Sperm need a specific temperature to survive. The scrotum makes sure the temperature is right for sperm to survive. So, when it's really warm, the scrotum stretches and the testicles hang lower and away from the warmth of the body. When it's cold outside, the scrotum pulls the testicles close to the body to keep them warm.

It is normal for the balls (testicles) to hang unevenly, with one lower than the other. This actually helps them avoid knocking into each other. (Ouch!)