Testicles are part of the set of organs that make up a boy’s reproductive system.
The testicles are located inside a sac of skin, called the scrotum, which hangs just below the penis. The scrotum protects the testicles and keeps them at a steady temperature.
Testicles produce sperm. Sperm are the itty-bitty cells that carry half the genes a baby will inherit from its parents. Sperm created in the testicles can fertilize a woman's egg to create a baby. The testicles connect to a duct system, made of the vas deferens and prostate, which moves sperm out of the body through the penis.
Testicles can produce hormones, too. During puberty, the time when a child’s body grows into its adult shape, testicles produce some of the hormone testosterone.
Testicles are special to boys’ bodies. The most similar body part on a girl is the ovary, inside a girl’s body. Ovaries contain eggs, which carry the other half of the genes a baby inherits. Girls don’t produce sperm and don’t have testicles.