Testicles, also called testes or balls, are oval-shaped organs that sit in a sac that hangs behind the penis.
The main job of testicles is to make and store sperm and produce testosterone. Testosterone is the male hormone that's responsible for the changes that occur during puberty. Puberty is the time in life when your body begins to change and you start to look more and more like an adult.
After puberty is finished, the testicles will produce millions of sperm a day. Sperm mixes with a white, milky substance to make semen. Semen is what's released from the penis when you ejaculate.
Guys become fertile between the ages of 11 and 17, meaning they are able to ejaculate semen and get a girl pregnant.