CubeClicker.cs

// Talking to objects
// 1. Find the object
// 1.a Manually dragging in the editor (public variables only)
// 1.b GameObject.Find
// 1.c GameObject.FindWithTag
// 1.d GameObject.FindGameObjectsWithTag
// 
// 2. Find the component
// 2.a Builtin (.transform)
// 2.b Custom (GetComponent<ComponentName>())
// 
// 3. Call functions or set variables
// 3.a Call functions directly (if using GetComponent<>())
// 3.b Set variables directly (if using GetComponent<>())
// 3.c SendMessage/BroadcastMessage (if not using GetComponent<>())

using UnityEngine;
using System.Collections;

public class CubeClicker : MonoBehaviour {
  public GameObject otherObject;
  public GameObject scoreObject;

  void Start() {
    // alternatives to manual dragging
    scoreObject = GameObject.Find("Score"); // 1.b
    otherObject = GameObject.FindWithTag("SphereTag"); // 1.c
  }

  void OnMouseDown() {
    Debug.Log("You clicked the cube!");

    // builtin components
    // object  .component.function/parameter
    otherObject.transform.Rotate(0, 0, 100); // 2.a

    // custom components, calling function
    // object  . component                 . function/parameter
    otherObject.GetComponent<SphereLogic>().DoubleInSize(); // 2.b, 3.a

    // any component on otherObject with DoubleInSize function, run it
    otherObject.SendMessage("DoubleInSize"); // 3.c
    // any component on otherObject with TripleInSize function, run it, dont complain
    otherObject.SendMessage("TripleInSize", SendMessageOptions.DontRequireReceiver); // 3.c

    // custom components, setting variables
    // object  . component                 . function/parameter
    scoreObject.GetComponent<ScoreScript>().score += 100; // 2.b, 3.b

    GameObject[] allTheSpheres = GameObject.FindGameObjectsWithTag("SphereTag"); // 1.d
    for(int i=0; i<allTheSpheres.Length; i++) {
      // object       . function/parameter
      allTheSpheres[i].SendMessage("DoubleInSize"); // 3.c

      // object       . component                 . function/parameter
      allTheSpheres[i].GetComponent<SphereLogic>().DoubleInSize(); // 2.b, 3.a
    }
  }
}

ScoreScript.cs

using UnityEngine;
using System.Collections;

public class ScoreScript : MonoBehaviour {
  public int score;
}

SphereLogic.cs

using UnityEngine;
using System.Collections;

public class SphereLogic : MonoBehaviour {
  public void DoubleInSize(float amount) {
    // transform.localScale = new Vector3(8, 8, 8);
    transform.localScale *= amount;
  }

  public void TripleInSize() {
    transform.localScale *= 1.5f;
  }
}