function in a function error

everytime I try to get a funtion inside a function i get this error on the second function no matter which order I put them in. "(#,#) expecting ( found whatever" Why is that? Here's one of the scripts i'm having this problem

function OnGUI(){
GUI.Box(Rect(10,10,150,100), "Press E to enter turret");

function OnMouseOver() {

    Debug.Log ("computer here");

    }
}

and my error is (4,10): BCE0044: expecting (, found 'OnMouseOver'.

Functions declarations can not be inside function declarations; it's really that simple.

If there's something you're trying to do, which, by your logic, would necessitate it, edit the question to name that, and we can give you a solution that will compile.