Testicles are a type of organ called glands. (This makes them part of the body's "endocrine system".) The human body has many types of glands. The job of glands within a body is to make "chemical substances", and put them out into the body's system. A body has lots of different systems that keep it in good working order all the time, and that also provide for a body's special needs. Some of the body's special needs happen when a person gets angry, frightened or sick, or if the person wants to have sexual intercourse. At these times a body uses more of some types of "chemical substance", so the glands that make them work harder. 