Create a simple model class in Magento

Assume we have a Helloworld module under MyExtensions package, and we want to create a simple model class and a hello function in this model to print hello world.
1. Add the below in the global tag in Magehome/app/code/local/MyExtensions/etc/config.xml

<models>
  <helloworld>
     <class>MyExtensions_Helloworld_Model</class>
  </helloworld>
</models>

2. Create a file at Magehome/app/code/local/MyExtensions/Model/Utilities/Hello.php

<?php
class MyExtensions_Helloworld_Model_Utilities_Hello
{
	public function hello()
	{
		echo "Hello World!!!";
	}
}

3. Now we can call this hello function anywhere in Magento like this:

Mage::getModel('helloworld/Utilities_Hello')->hello();