[NickTheGreat]

I'm new to flash/actionscript, and I'm attempting to make a pretty simple flash platform game. I got my character easily moving left and right, but I can't figure out jumping.
I tried:

if(Key.isDown(Key.SPACE)){
velocity = 17;
_y = _y + velocity;
}

but that makes him fly... so I tried adding :

if(_y < *input height here*){
velocity = -17;
}

but if the player lets go of the spacebar in midair, the character doesn't fall... can anyone help???