title: cannot assign to value: 'word' is a 'let' constant
body: <p>I get following error: "cannot assign to value: 'word' is a 'let' constant"
  I can't figure out why this happens.
  If someone could help me, I would appreciate it a lot.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
  
      var word1 = ""
      @IBOutlet weak var label: UILabel!
  
      func writeWord(word: String){
          word = "Example"
      }
  
      @IBAction func button(sender: AnyObject) {
          writeWord(word1)
          label.text = word1
      }
  
      override func viewDidLoad() {
          super.viewDidLoad()
          // Do any additional setup after loading the view, typically from a nib.
      }
  
      override func didReceiveMemoryWarning() {
          super.didReceiveMemoryWarning()
          // Dispose of any resources that can be recreated.
      }
  }
  </code></pre>