haring application might have a <code>Photo</code>
model to represent a particular photo, and a <code>PhotoAlbum</code> that
represents a group of photos. In contrast, an online shopping app would
probably have different models, like <code>ShoppingCart</code>, <code>Invoice</code>, or
<code>LineItem</code>.</p>
<p>Models tend to be <em>persistent</em>. That means the user does not expect
model data to be lost when they close their browser window. To make sure
no data is lost, if the user makes changes to a 