Learn Responsive Web Design by Building a Piano - Step 6

how do i add miltiple class attribute within an element?

  **Your code so far**
/* file: index.html */
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <title>Piano</title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
</head>
<body>
  <div id="piano">
    <div class="keys">
      <div class="keys" class="black--key">
      </div>
      <div class="keys" class="black--key">
      </div>
      <div class="keys">
      </div>
      <div class="keys" class="black--key">
      </div>
      <div class="keys" class="black--key">
      </div>
      <div class="keys" class="black--key">
      </div>
      <div class="keys">
      </div>
    </div>
  </div>
</body>
</html>
/* file: styles.css */

  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Safari/537.36

Challenge: Learn Responsive Web Design by Building a Piano - Step 6

Link to the challenge:

You separate the class names with a space.