The scrotum is the sack of skin that hangs below the penis. Inside the scrotum are the testicles (testes), which are two small organs that are sometimes called “balls.” It is normal for the testicles to hang unevenly, with one lower than the other (usually the left!). This actually helps them avoid knocking into each other. (Ouch!)

Sperm cells are made in the testicles, then stored in tubes nearby that are called the epididymis. Sperm need a specific temperature to survive; one that is lower than body temperature. Because of this, the scrotum hangs outside of the body in order to regulate the testicular temperature. That’s why when it's really warm, the scrotum relaxes and allows the testicles to hang lower, away from the warmth of the body, and when it's cold outside or when you’re in cold water, the scrotum pulls the testicles close to the body to keep them warm.

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.