When you reach puberty, you may start noticing a different fluid coming out of the penis - one that is milky in colour and is sticky. It is thick, not watery like urine.
Ejaculation is the climax of sexual excitement, when a man has an erection.
Keep in mind that you don't have to ejaculate every time you have an erection. If you just wait, the erection will go down on its own without causing any harm to you. The mucus-like fluid is made up of two things. Sperm are so small that you cannot see them unless you have a microscope. If you could see them, you would see that each sperm has a round body or head, and a long thing tail.