In one of my sites I chose to use the menu "main menu", and use it in the region "navigation". This was all good, until I discovered that the child of each menu link was not displayed if I clicked the parent. It was not automatically expanded on the page I clicked.
Say you want to have a menu structure like this:
about us services -animated gifs -background midi music contact
Where the menu items “animated gifs” and “background midi music” are sub menus, or children, of the menu item services. If you add these menu items to the main menu and activate the block “main menu” in a region, those sub items will not appear when you click “services”, as you may often want. The menu item simply does not expand when clicked. I don't know why, as I personally think this should be the default behavior, and for a while I was searching for what I was missing in the default settings. Ah, this should be so simple, I thought, and of course it was. The answer: There's a module for that! Menu block.
And of course a couple of other solutions, but why not just go for the simple solution here? I know I did. Let's celebrate with an animated GIF: