An erection or hard-on is when you become sexually excited. Extra blood flows into the penis making it stiff and stand out from your body. The extra blood makes it feel warmer and look redder.
Erections can happen at any time and during puberty the penis seems to develop a mind of its own. Sometimes you can get an erection by seeing something sexy on TV or someone you are attracted to. Other times there is no reason at all. This can be embarrassing although it's important to remember it's normal and happens to everyone.
Unexpected hard-ons will go away after a while. You can try and help this happen by thinking about something non-sexual or even frightening.
