title: Div doesn't have height unless I give it min-height
body: <p>I think this is because I'm floating all three divs "columns" to the left, inside of the main body div.</p>
  
  <p>How can I tell the main body div to expand as big as it needs to fit the content divs?</p>
  
  <p>Here it's with min-height:
  <img src="https://i.stack.imgur.com/F3Ow4.jpg" alt="alt text"></p>
  
  <p>And here with the min-height taken away:
  <img src="https://i.stack.imgur.com/Tr9T1.jpg" alt="alt text"></p>
  
  <p>Here is my relevant code.</p>
  
  <pre><code>#body
  {
      border:1px solid blue;
      width:950px;
      margin-left:auto;
      margin-right:auto;  
      margin-top:15px;
  }
  
  #leftcolumn
  {
      min-height:500px;
      float:left;
      width:190px;
  }
  
  #contactarea
  {
      font-family:Arial;
  }
  
  #contactarea p.heading
  {
      Color:#000;
      font-size:large;
      position:relative;
      left:14px;
  }
  
  #contactarea p.tag
  {
      color:#000;
      font-size:medium;
      position:relative;
      left:10px;
  }
  
  #leftnavigation ul
  {
      margin:0;
      padding: 0;
      list-style: none;
  }
  #leftnavigation ul li {
      border-top: 1px solid #333;
      border-bottom: 1px solid #111;
  }
  #leftnavigation ul li:first-child {border-top: none;}
  #leftnavigation ul li:last-child {border-bottom: none;}
  #leftnavigation ul li a 
  {
      padding: 10px;
      display: block;
      color: #fff;
      background-color:#222222;
      text-decoration: none;
  }
  #leftnavigation ul li a:hover {background: #111;}
  
  #contentarea
  {
      border:1px solid blue;
      min-height:500px;
      float:left;
      width:594px;
      margin-left:5px;
      margin-right:5px;
  }
  
  #advertisingarea
  {
      width:150px;
      float:left;
      min-height:500px;
      background-image:url('images/advertisingAreaBackground.png');
      background-repeat:repeat-y;
  }
  
  .advert
  {
      height:190px;
      overflow:hidden;    
  }
  
  .advert img
  {
      padding:0;
      margin:0;
      position:relative;
      left:25px;
      top:5px;
  }
  
  .advert p
  {
      font-size:x-small;
      font-family:Arial;
      margin-left:8px;
      margin-right:8px;
      margin-top:5px;
  }
  
  &lt;div id="body"&gt;
      &lt;div id="leftcolumn"&gt;
          &lt;div id="leftnavigation"&gt;            
              &lt;ul&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Automoviles&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Clases y Talleres&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Electronicos&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Inmobiliaria&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Libros&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Musica, Peliculas y Juegos&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Ninos&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Otros&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;Ropa&lt;/a&gt;&lt;/li&gt;
              &lt;/ul&gt;
          &lt;/div&gt;
          &lt;div id="contactarea"&gt;
              &lt;p class="heading"&gt;Anuncios Premium&lt;/p&gt;
              &lt;p class="tag"&gt;Destaque sus anuncios con una cuenta premium!&lt;/p&gt;
          &lt;/div&gt;
      &lt;/div&gt;
      &lt;div id="contentarea"&gt;sdfg&lt;h1&gt;asdasd&lt;/h1&gt;&lt;/div&gt;
      &lt;div id="advertisingarea"&gt;
          &lt;div class="advert"&gt;
              &lt;a href="#"&gt;&lt;img src="../../Content/images/advertImage.png" alt="Advert" /&gt;&lt;/a&gt;
              &lt;p&gt;Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris nibh nisi, volutpat a vehicula eget&lt;/p&gt;
          &lt;/div&gt;
  
          &lt;div class="advert"&gt;
              &lt;a href="#"&gt;&lt;img src="../../Content/images/advertImage.png" alt="Advert" /&gt;&lt;/a&gt;
              &lt;p&gt;Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris nibh nisi, volutpat a vehicula eget&lt;/p&gt;
          &lt;/div&gt;
  
          &lt;div class="advert"&gt;
              &lt;a href="#"&gt;&lt;img src="../../Content/images/advertImage.png" alt="Advert" /&gt;&lt;/a&gt;
              &lt;p&gt;Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris nibh nisi, volutpat a vehicula eget&lt;/p&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>