The main man glands. Usually exist in pairs. Testicles are useful in development, reproduction, testosterone production, and pocket pool. They are housed in the scrotum, where the right testicle hangs lower than the left testicle in 85% of men. Testicles work better when they are approximately 5.4 degrees Fahrenheit (4 degrees Celsius) cooler than normal body temperature, so nature designed a dual pulley system (the cremaster and dartos muscles) to lower them and raise them depending on whether the external temperature is warmer or cooler than what is optimal. The testicles of a healthy young man produce between 100 and 300 million sperm a day. However, contrary to what most people understand, the testicles only produce a couple of drops (less than 5%) of the total semen volume. People also think that the testicles release testosterone in semen, when that is not the case. The testicles release testosterone into the blood stream, so a partner is not going to get a mouthful of testosterone when giving a guy a blowjob. Cancer of the testicles is the most common cancer in young men. Fortunately, it is 95% curable if treated early.