It’s fair to say most men have experienced blue balls at least once. Yeah, they were probably sexually frustrated at the time — I can’t think of any other time it would happen. And no, it’s not just a term for sexual frustration. Blue balls is totally real.

Most men can also recount how it happened. Some rendition of making out that led to foreplay that led to… nothing. “Maybe next time,” she said. All that sexual arousal and no orgasm. Like an overheated thermometer ready to burst. Men who get blue balls usually complain about intense pressure, heaviness, aching, and discomfort.

So, what are blue balls? As a man becomes sexually aroused, the arteries carrying blood to his genitals enlarge, while the veins carrying blood away constrict. This traps the blood in the genitals, helping him maintain an erection. It also swells the testes to 25 to 50 percent larger than their normal size. Soon, the man will reach orgasm and everything in the genitals will return to their normal state. Unfortunately, in this case, he doesn’t. Blood pressure continues to build in the genitals through a process called vasocongestion, which makes it feel like someone is squeezing the testes.

Though the testes certainly feel bruised and blue at this point, the term comes from the bluish hue that develops on the testicles, according to the University of California, Santa Barbara. Oxygen-rich blood passing through the body is red. But when it loses oxygen — from being trapped in the testicles — it begins to turn blue. Hence, blue balls.

While this may be the prevailing science behind why we get blue balls. Dr. Abraham Morgentaler, director of Men’s Health Boston and associate clinical professor of urology at Harvard Medical School, told BuzzFeed there may be another reason: Fluids produced in the testicles, which are meant to propel sperm out of the penis, build pressure.

“With more stimulation, there’s more blood flow to the genital region, including the testicles,” he said. “The fluid wants to get somewhere, but without there being that release, it builds up and builds pressure.” As the fluid builds up, it causes the tubes at the back of the testicles to stretch, causing pain. “It has that kind of sensitivity and tenderness.”

Having an orgasm and ejaculating is the easiest way to find relief from blue balls, and since it’s not going to come from having sex, masturbation might have to suffice. Once done, the pain and swelling associated with blue balls should go down. If masturbation isn’t an option, however, the pain will eventually still subside as the blood and fluids drain out. It’ll just take a bit longer.