When I go swimming my scrotum shrinks. How come?

For sperm production the temperature of the testicles needs to be lower (93 degrees Fahrenheit) than the body temperature. That’s why they hang outside the body. The scrotum works as a thermostat. When it’s cold the scrotum contracts upwards towards the body to warm the testicles. When it’s warm the muscles relax and the scrotum drops, becoming loose and soft, causing the testicles to hang farther from the body to cool off. The scrotum also contracts when you’re afraid, protecting the testicles against harm in case of an attack. So it’s perfectly normal that your scrotum shrinks when you are cold: it’s well functioning.