The testicles have two main jobs: to make hormones and sperm. The testicles make the hormone testosterone, which spreads around the body. This causes the changes in the body that make a boy grow into a man. Testosterone also makes a man feel like having sex. One year after puberty begins, when boys urinate (pee) after waking up in the morning, some sperm can be found in the urine. When a boy is about 13 years old his testicles will have grown to the stage that he may be fertile (able to make a woman pregnant). However, it will be still be difficult for him to do so. Boys usually only become fully fertile when they reach about 14 to 16 years of age. The sperm in the testicles mixes with other liquids in the body to form a whitish or yellowish and sticky fluid called semen. During puberty, two glands in the body called the seminal vesicles and prostate also grow bigger. These glands make a lot of the liquid that is in semen.