Pre-ejaculate (also known as pre-ejaculatory fluid, preseminal fluid, or Cowper's fluid, and colloquially as pre-cum) is the clear, colorless, viscous fluid that emits from the urethra of a man's penis when he is sexually aroused. It is similar in composition to semen, but has some significant chemical differences. The presence of sperm in the fluid is debated. Research has found no or low levels of sperm in pre-ejaculate, though these existing studies are non-generalizable due to examining small numbers of men. A contrary, yet non-generalizable study that found mixed evidence, including individual cases of a high sperm concentration, has been recently published. Pre-ejaculate is believed to function as a lubricant and an acid neutralizer. The amount of pre-ejaculate emitted varies widely between individual men. There are some cases where men do not pre-ejaculate.