The testicles are the two small ‘balls’ (slang, offensive) in the scrotum. They are part of the man’s external genitals. They produce sperm cells and the male sex hormone testosterone. An epididymis lies beside each testicle, this is where the sperm cells are stored before they come out into the spermatic cord.
You should go to the doctor if you notice a lump on a testicle. See testicular cancer.