if with two conditions not working

I've been working on unity scripting a few days ago and I'm still a beginner so excuse my poor scripting skills I am working on a project where clicking on the "SunRise" gameObject disables the "funnyMoon" gameObject and when clicking on the "letter" gameObject it will be ddestroyed if it is tagged "Sun" Here are the scripts

 using UnityEngine;
 using System.Collections;
 
 public class SunBoxBehaviour : MonoBehaviour {
 
     // Use this for initialization
     public int IsSun = 0;
     public GameObject funnyMoon;
 
     void OnMouseDown()
     {
         IsSun++;
         funnyMoon.SetActive (false);
         Debug.Log(IsSun);
     }

Here is the second script:

 using UnityEngine;
 using System.Collections;
 
 public class LetterBehaviour : MonoBehaviour {
 
 
     public SunBoxBehaviour SunBoxBehaviour;
     public GameObject SunRise;
 
     void OnMouseDown()
     {
         if (this.gameObject.tag == "Sun")
         {
             Debug.Log ("This is Sun");
             if(SunBoxBehaviour.IsSun == 1) 
             {
             Destroy (this.gameObject);
             Debug.Log ("You made it");
             }
         }
     }
 

the issue is that it is checking for the first condition: if the letter tagged "Sun" but it's not checking for the other condition. Why is that?