An experimental male contraceptive method involves heating the testicles so that they cannot produce sperm. Sperm are best produced at a temperature slightly below body temperature. The muscles around a male's scrotum involuntarily tighten if the man's body temperature drops, and they loosen, allowing the testes to hang, if the body temperature rises. This is the body's way of keeping the sperm at an ideal temperature. Although research has not addressed methods of applying heat, sperm production can be disrupted with increased temperature. Some suggest exposure to high temperatures (116 °F) can affect fertility for months.