I sometimes wake up with a wet, sticky spot on my underwear or pajama pants. It's not urine, so what is it? - Stewart*
Chances are that you had a "wet dream" — something that can be embarrassing and confusing, but is completely normal. A wet dream is also known as a nocturnal emission. Nocturnal means "at night" and emission means "discharge." This makes sense because a wet dream is when semen (the fluid containing sperm) is discharged from the penis during ejaculation while a guy's asleep. Usually this happens during dreams that have sexual images. Sometimes guys wake up from a wet dream, but sometimes they sleep through it.
Wet dreams begin during puberty when the body starts making more testosterone, the major male hormone. Although some guys may feel embarrassed or even guilty about having wet dreams, they can't be controlled and you can't stop them from happening — most guys experience them at some point during puberty and even sometimes as adults. Wet dreams usually happen less as guys get older, though.