void OnCollisionEnter2D(Collision2D col)
{
//there is a new collision, we must notify the gameManager
_pool.ReturnItem(this.gameObject); //we return the plane to the pool
col.transform.GetComponentInParent <PoolManager>().ReturnItem(col.gameObject); //we return the bullet to the pool
_generator.PlaneDestroyed(); //we notify the generator that the plane is destroyed
}