Usually between the ages of 11 and 15; boys may experience their first ejaculation, a landmark of achieving adulthood. Ejaculation is the process by which semen/Sperms are ejected from the penis.
Sperms are very tiny live seeds for reproduction, produced in testicles. Penis have only one opening. Urine also comes out from the same urinary opening from the tip of the penis. When urine is coming out, ejaculation of sperms will not happen.
Before a male ejaculates, he gets an erection. Blood fills the tissues inside the penis so that the penis stiffens and becomes thicker, longer and harder, which is called ERECTION.
Some clear liquid may also comes out before final ejaculation. When a male ejaculates, muscles contract and push semen/sperms into & through the urethra and comes out from opening at the tip of the penis.
The semi-liquid type mucous will come out which is very sticky. An ejaculation that happens while a boy is asleep is called a Nocturnal Emission or WET DREAM.
It may be the result of sexual fantasy in the dream. It usually occurs involuntarily, which are normal. Only, it may stain the underwear or nightdress. Pics