how to mark two objects and collision to explode and play the sound

Hi, how do I tag the second object in the code below? Code: public class DestroyByContact2 : MonoBehaviour {

 public AudioClip impact;
 AudioSource audioSource;
 public GameObject explosion; // drag your explosion prefab here

 void Start()
 {
     audioSource = GetComponent<AudioSource>();
 }


 
 void OnCollisionEnter(Collision collision)
     {

     if (collision.gameObject.tag == "Asteroids" ) { 
     
         GameObject expl = Instantiate(explosion, transform.position, Quaternion.identity) as GameObject;


         audioSource.PlayOneShot(impact, 0.7F);
         Destroy(collision.gameObject); // destroy the grenade
         
         Destroy(expl, 5); // delete the explosion after 2 seconds
          }
     }
      

}