why do i keep getting an error?

so i am trying to make objects appear in different places with this script. i keep on getting an error! so what am i doing wrong?

function Update ()
{var prefab : GameObject;
function Start () {
var position = Vector3(Random.Range(-10, 10), 0, Random.Range(-10, 10));
Instantiate(prefab, position, Quaternion.identity);
}
}

your running the Start function in an update function, that will not work.
try this :

var prefab : GameObject;
function Start () {
var position = Vector3(Random.Range(-10, 10), 0, Random.Range(-10, 10));
Instantiate(prefab, position, Quaternion.identity);
}