When a guy starts going through puberty, the first thing that happens is growth of the testicles. All of the other changes happen as a result of the increased testosterone, a hormone produced by the testicles.

The first thing a guy usually notices is hair growing under his arms, around his penis area (genitals) and on his face. Hair on other parts of his body will often get thicker and darker too. Some guys have more hair than others. Genetics play a part in this. That means if your biological dad, for example, has a lot of body hair, you might have a lot of body hair too.

Also during puberty, a guy's voice box grows. That's what causes his voice to crack and squeak. It takes a little time for the cracking to stop, as the voice deepens and settles into its adult tone. The penis, testicles and scrotum also grow during puberty. The testicles will also start to produce sperm. Since your testicles are growing faster than your penis at first, it might seem like your penis is shrinking. It's not. In time, it all evens out.

During puberty, it's also common for boys to have large growth spurts, times when they grow fast over a short period of time. This rapid growth can cause "growing pains," when joints and muscles ache and get sore. Also, when a guy grows very fast, he doesn't have time to get used to his larger limbs, so he sometimes feels awkward and clumsy.

Another thing that can happen to guys during puberty is getting an erection out of nowhere. Erections that happen for no reason at all are called “spontaneous erections.” Teen guys can get erections when they don’t even feel turned on. They can happen without warning and at embarrassing times. Most teen guys who are going through puberty get spontaneous erections. They happen a lot less once a guy finishes puberty, although adult men still get them every once in a while.