Sperm are produced in the testes, the male reproductive organs. The testes are located outside the main body cavity in the scrotum, a pouch of loose skin below the abdomen. The testes also produce the male sex hormone testosterone. Boys start producing sperm at puberty (approximately 15 years of age), and most men have mature sperm well into old age. Sperm are made at the rate of about 300 million per day, and live about 48 hours after being deposited in the woman's vagina. Sperm are equipped with a tail which propels them along and helps them to be carried by fluid movement up the female reproductive tract (i.e. the vagina, cervix, and fallopian tubes) to reach and penetrate the ovum. The sperm pass out of the testes into the male reproductive ducts where they are suspended in secretions which form semen. The average volume of semen for each ejaculation is 2.5 to 6 mls, and the average number of sperm ejaculated is 40-100 million per ml.