Collision Detection between two different Prefabs doesnt work

So i have two Prefabs (on of them is a Bullet, the other one is a standing Object spawned "by the player"), w$$anonymous$$ch i want to collide and get destroyed. When i put the script down below on my Player and run into the spawned Object with the Tag "Shot Target" it gets destroyd, but when i have the Script on my bullet, there seems to be no Collision between the both. Anybody got an Idea?

 void OnCollisionEnter(Collision col)
 {
     if (col.gameObject.tag == "ShotTarget")
     {
         Destroy(col.gameObject);
     }
 }