4 CHAPTER 1 Writing a simple application Because MXML files are ordinary XML files, you have a wide choice of development environments. You can write MXML code in a simple text editor, a dedicated XML editor, or an integrated development environment (IDE) that supports text editing. Flex supplies a dedicated IDE, called Adobe® FlexTM BuilderTM, that you can use to develop your applications. The following example shows a simple "Hello World" application that contains just an <mx:Application> tag and two child tags, the <mx:Panel> tag and the <mx:Label> tag. The <mx:Application> tag defines the Appli- cation container that is always the root tag of a Flex application. The <mx:Panel> tag defines a Panel container that includes a title bar, a title, a status message, a border, and a content area for its children. The <mx:Label> tag represents a Label control, a very simple user interface component that displays text. <?xml version="1.0"?> <!-- mxml\HellowWorld.mxml --> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"> <mx:Panel title="My Application" paddingTop="10" paddingBottom="10" paddingLeft="10" paddingRight="10"