How to make health go down when objects collide

Here's my script

/Health Script/

var clockBG : Texture2D; var clockFG : Texture2D; var clockFGMaxWidth : float; //the starting width of the foreground bar var currentHealth:float;

function Start() { startTime = 10.0; clockFGMaxWidth = clockFG.width; currentHealth = 95.8; }

function Update () { if (Input.GetButtonDown("Fire1")) currentHealth -= 1; }

function OnGUI() { //this is the width that the foreground bar should be var newBarWidth:float = (currentHealth/100) * clockFGMaxWidth;

 Debug.Log("current health " + currentHealth);
 
 //a spacing variable to help us position the clock
 var gap:int = 20;
 
 GUI.BeginGroup(new Rect (Screen.width - clockBG.width - gap, 
    gap, clockBG.width, clockBG.height));
    GUI.DrawTexture(Rect (0,0, clockBG.width, clockBG.height), clockBG);
 
    GUI.BeginGroup(new Rect(5,6, newBarWidth, clockFG.height));
      GUI.DrawTexture(Rect(0,0, clockFG.width, clockFG.height), clockFG);
    GUI.EndGroup();
 GUI.EndGroup();
 }

I was wondering how do I make the health drop when an object collides with another, sort of like when two enemies hit, they attack each other. Let me know if I need to post any more information to help you answer the question.