Why is the temperature of scrotal sac less than body temperature?

The scrotum is a pouch of skin that contains the testes. It is located outside the body, and as such, it is exposed to the ambient temperature. The testes need to be kept at a temperature that is slightly lower than body temperature in order to produce sperm properly. This is why the scrotum is able to contract and relax, moving the testes closer to the body or further away from it, depending on the temperature.