help in converting from keyinput to collsion script

hi this script is work for my bullet .when i press a button it will clone sphere (one bullet).but i want to change the code like the sphere want to emit after it collsion with other object like cube .please help me to do this:thanks in advance

Code (csharp):

1.  
2. using UnityEngine;
3. using System.Collections;
4.  
5. public class pra : MonoBehaviour{
6.  
7. public GameObject gun;
8. private GameObject bullet=null;
9. private float power;
10.  
11.  
12. void Start()
13. {
14.  
15. bullet=GameObject.Find("bullet");
16.  
17.  
18. }
19. void Update()
20. {
21. if(Input.GetKeyUp("a"))
22. {
23.              power =10;
24. GameObject obj = Instantiate(bullet, gun.transform.position, gun.transform.rotation) as GameObject;
25.             obj.rigidbody.velocity = gun.transform.TransformDirection(Vector3.left*power);
26.             obj.name = "BulletClone";
27.            
28.  
29. }
30.        
31.        
32.        
33.  
34. }}
35.  

 

Put the instantiation code in OnCollisionEnter instead.