Testicles, also called testes, are two small oval shaped organs. They are the male sex glands which hang down behind the penis, in the scrotum (the small muscular sac). They produce sperm and 90% of testosterone.
It is quite normal for one testicle to be slightly larger than the other or hang lower, although the size and shape should be roughly the same. Testicles start growing at the age of about 11 or 12 and will reach their final size by early adulthood.