diff --git a/Structural/Adapter/PaperBook.php b/Structural/Adapter/PaperBook.php
index f8ad3c4..c9ebf35 100644
--- a/Structural/Adapter/PaperBook.php
+++ b/Structural/Adapter/PaperBook.php
@@ -6,12 +6,12 @@ class PaperBook implements Book
 {
     private int $page;
 
-    public function open()
+    public function open(): void
     {
         $this->page = 1;
     }
 
-    public function turnPage()
+    public function turnPage(): void
     {
         $this->page++;
     }