title: Persistent value of instance variable
body: <p>I've created the following class</p>
  
  <pre><code>class BankAccount                                                       
    def accountNumber                                                   
      @accountNumber=5                                                
    end                                                                 
    def accountNumber=(value)                                           
      @accountNumber=value                                            
    end                                                                 
  end    
  </code></pre>
  
  <p>and I use it like this:</p>
  
  <pre><code>account=BankAccount.new
  =&gt; #&lt;BankAccount:0x0000000295d6c8&gt;
  account.accountNumber
  =&gt; 5
  account.accountNumber="223"
  =&gt; 223
  account.accountNumber
  =&gt; 5
  </code></pre>
  
  <p>why is <code>accountNumber</code> equal to <code>5</code> even after setting it to <code>223</code>?</p>