Testicles (also called the testes; a single testicle is called a testis) are part of the male reproductive system. These 2 organs are each normally a little smaller than a golf ball in adult males and are contained within a sac of skin called the scrotum. The scrotum hangs beneath the base of the penis.
Testicles have 2 main functions:
They make male hormones (androgens) such as testosterone.
They make sperm, the male cells needed to fertilize a female egg cell to start a pregnancy.
Sperm cells are made in long, thread-like tubes inside the testicles called seminiferous tubules. They are then stored in a small coiled tube behind each testicle called the epididymis, where they mature.