How do I exucate an action only once when a key is pressed constantly?

1

could somebody helpme with this one?

I would like to perform an action only once when I’m holding down a key.
In my case the arrow keys. The problem is when I place the statments
inside the Keylistener functions it exucate them constantly.
So. How do I exucate them only once while the key is not released?
here is the script

if (Key.isDown(Key.LEFT)) {
_root.car_mc.attachMovie(“MCball”, “ball_mc”, 1);

}