ANDY'S ANSWER:  Unexpected erections are unfortunately one of the things that can cause embarrassment for a boy going through puberty. Often, nervousness or even a full bladder can bring on an erection when you least want or expect it. There are a couple of things you can do so that it's not so obvious when it does happen. Some guys wear a jock-strap to keep their tiger under control, but wearing snug underwear under loose pants may be enough to tame your beast. A loose-fitting longer shirt may also give you some protective cover.