Semen is the white fluid that comes out of the penis when the man has an ejaculation. Semen consists of sperm and other fluids such as seminal fluid. Sperm cells are produced in the testicles, located in the scrotum. A sperm cell is 0.06 millimetres long, and has a long thin tail helping it to swim in the fluid. The rest of the fluid is produced by the seminal vesicles and the prostate gland. This fluid nourishes the sperm cells and helps them to move around more easily when they enter the woman’s vagina. There can be as many as 350 million sperm cells in one ejaculation.