Joe Wilfried Tsonga was incapacitated by Andy Murray during the 2012 Wimbledon Semi-Finals. YouTube/SchumakerPole69 More than any other bodily injury, getting hit in the testicles is probably what every man dreads most.

Of all the soft, fleshy spots on the human body, none register the same kind of incapacitating, end-of-the-world pain as the family jewels.

What causes such inconceivable pain? Well, for starters, a shot to the balls is just like any other physical strike to the body: because of nerves, it’s gonna hurt. Unlike most other parts of your body, though, your scrotum lacks protection in the form of bones, large muscle mass, and fat. The testes are just wee little glands, and they’re going to absorb the whole force of the blow all on their own.

Another thing that makes a ball shot so painful is the same thing that makes almost every other sensation down there so much fun. Your groin has a ridiculously high number of sensory nerve endings, and such generous innervation makes good and bad touches alike very noticeable sensations.

And the pain doesn’t just stay down there in the scrotum. It insists on radiating throughout the groin and up into the abdomen (and, psychically, out to every other dude standing within a few feet), leading to a weird stomach ache. This is the work of a phenomenon known as referred pain, which is when a sensation originating at one spot travels along a nerve root to other parts of the body and is perceived as happening there, too. It’s the same thing that’s going when you get an ice cream headache. In this case, the pain starts in your balls and travels up the perineal and pudendal nerves and the spermatic plexus, which cover real estate in the groin and abdomen, around the spine and even a little ways down into the anus, to make it feel like death has come for most of your lower body.

Location, Location, Location

Why is such a sensitive and delicate body part just hanging there in the open? The placement of the testicles is inconvenient, but absolutely necessary. The testes’ job is to produce sperm, and sperm are very fragile. They’re extremely sensitive to high and low temperatures, and must be kept away from the rest of the body and relatively exposed to maintain the right climate. They can handle human body temps for only one to four hours, or the average amount of time it takes them to travel through the female reproductive tract and fertilize an egg. Internal testes or any type of significant shielding for them would heat them up too much, too early and make them drop out of the race well before reaching the egg, rendering them useless.

The scrotum isn’t just a dumb sack swaying in the breeze, though. In deference to our genetic interests, our bodies subconsciously thermoregulate our balls by flexing the cremasteric muscle and drawing the scrotum up closer to the body when it gets too cold and dropping it when it’s hot. This optimized, on-the-fly sperm storage is precise enough that each testicle can be repositioned independent of its twin in order to get the temperature just right, explaining their sometimes asymmetrical dangle.