The testicles (also known as testes) are the male sex glands and are located in the scrotum. Testes produce testosterone, but optimal production depends on the right environment, which means you need to keep your testicles cool, about 2 degrees cooler than the rest of the body.
If you wear tight trousers, underwear, or biking shorts, ride a bicycle, or otherwise engage in activities that keep your testicles crowded or up tight against your body, then your testicle temperature can rise and you risk a reduction in the production of testosterone. Therefore, be conscious of what you do and wear and dress for success (of T production).