Your testicles hang behind your penis in a bag of skin called the scrotum.

Every boy’s scrotum looks different. It can be large, small, wrinkled or quite smooth, hairy or bald. One testicle usually hangs a bit lower than the other, and sometimes they’re slightly different sizes.
 

Testosterone
Inside the scrotum there are two testicles, or testes. Your testicles produce and store millions of tiny sperm cells. They also produce testosterone, a hormone that causes boys to produce sperm and develop bigger muscles, deeper voices, and body hair. One thing is for sure, the testicles are very sensitive and easily damaged. So a word of advice: never kick anyone there.

Image:
1. Testes
2. Scrotum

Temperature
When it’s cold, your scrotum shrinks. When it’s hot, your scrotum gets bigger. This happens because to make sperm, your testicles need to be at just the right temperature – a bit cooler than the rest of your body. So when it’s cold your scrotum squeezes your testicles up close to your body to keep them warm, and when it’s hot your scrotum lets your testicles hang down to keep cool. This happens without you thinking about it!