Eli5-bouncing testicles

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Why do testicles bounce during sex, anal sex/anal masturbation or just masturbation in general? Is there a biological reason to this?

I really don’t understand that part of my body at all.

In: Biology

No there is no biological reason for this. This is basic inertia – a physics thing: An object in motion prefers to continue moving in the same direction. To change its direction you have to neutralise the motion it already has, first. You can experience inertia for yourself when you are sitting in a car and you drive around a tight corner quite quicky: You sway in the same direction as you were travelling before you turned, because it takes your body a moment to catch up with the turn that the car has made.

Same principle operates on your testicles. When you move forward, your balls move forwards too, but when you move backwards, your balls take a moment to catch up: They’re free-swinging, so they don’t immediately lose all momentum they already have when you make them change direction, they have to take the time to lose this momentum, which means they hang in the air for a bit before being dragged in the new direction.

Testicles are located outside of the body because the process to create ~them~ sperm requires a temperature slightly below normal body temp. Muscles can tighten or loosen to pull the scrotum taught or loose, which in turn raises the testicles and holds them close to the body, keeping them warmer, or lowering them away from the body to cool.

During vigorous activity, including sex, testicles tend to retract and hold close so they *don’t* bounce around too much. However, vigorous activity also tends to raise your body temperature, which is dangerous to the sperm so the testicles have to lower to stay cool.