diff --git a/src/Gun.js b/src/Gun.js
index 5f7287a..729820f 100644
--- a/src/Gun.js
+++ b/src/Gun.js
@@ -13,12 +13,12 @@ The Gun is the mouse. To draw it, hide the cursor with the noCursor function and
 
 
 class Gun extends Subject {
-  constructor(totalShots) {
+  constructor(totShots) {
     super();
     this.cursorImg = null;
     this.shotSound = null;
     this.emptySound = null;
-    this.remainingShots = totalShots;
+    this.remainingShots = totShots;
     this.bullets = [];  // array of Bullet objects
   }
 
diff --git a/src/ScoreDisplay.js b/src/ScoreDisplay.js
index ae804a2..7cffb31 100644
--- a/src/ScoreDisplay.js
+++ b/src/ScoreDisplay.js
@@ -38,6 +38,16 @@ class ScoreDisplay {
     }
 
   }
+
+  update(source, ...others) {
+    if (source === 'gun') {
+      const [, , remainingShots] = others;
+      this.shotLeft = remainingShots;
+    } else if (source === 'target-hit') {
+      const [points] = others;
+      this.score += points;
+    }
+  }
 }
 
 export { ScoreDisplay };
diff --git a/src/Subject.js b/src/Subject.js
index 746a9ff..4defcce 100644
--- a/src/Subject.js
+++ b/src/Subject.js
@@ -1,5 +1,27 @@
 class Subject {
-  // TO DO
+  constructor() {
+    this.observers = [];
+  }
+
+  subscribe(observer) {
+    if (!observer || typeof observer.update !== 'function') return;
+    this.observers.push(observer);
+  }
+
+  unsubscribe(observer) {
+    this.observers = this.observers.filter(o => o !== observer);
+  }
+
+  unsubscribeAll() {
+    this.observers = [];
+  }
+
+  notifySubscribers(source, ...others) {
+    for (let observer of this.observers) {
+      observer.update(source, ...others);
+    }
+  }
 }
 
+
 export { Subject };
diff --git a/src/Target.js b/src/Target.js
index 833c72a..ac723e8 100644
--- a/src/Target.js
+++ b/src/Target.js
@@ -39,6 +39,15 @@ class Target extends Subject {
       py >= this.y &&
       py <= this.y + TARGET_WIDTH;
   }
+
+  update(source, ...others) {
+    if (source === 'gun') {
+      const [x, y] = others;
+      if (this.contains(x, y)) {
+        this.hit();
+      }
+    }
+  }
 }
 
 class TeddyTarget extends Target {
diff --git a/src/main.js b/src/main.js
index 43a5273..fdfc9ac 100644
--- a/src/main.js
+++ b/src/main.js
@@ -20,6 +20,14 @@ function setup() {
   // 2. Init the targets
   initTargets();
   // 3. Subscribe gun
+  gun.subscribe(score);
+
+  // Subscribe each target to gun
+  for (let target of targets) {
+    gun.subscribe(target);
+    target.subscribe(score);
+  }
+
 }