By the way, if you put your cursor on the Start Button and do nothing -- no right clicking, no left clicking, in fact take you hand off the mouse just to be sure -- something will happen. A little yellow box will pop up with the words: "Click here to begin."
It's a ToolTip. Frequently in Windows letting your cursor hover over a button will call forth a ToolTip that tells you, or at least hints at, what clicking on the button would do.
Sometimes a TootTip is very specific, like "Open new side note" (for the OneNote icon in the System Tray). Sometimes it's a little more general, like "Mozilla Firefox" (for an icon on the QuickStart Menu). Sometimes it's downright vague (begin what?). In any case, ToolTips are there to help and they assume you need help when your put your cursor on something and hesitate before clicking.
You may have noticed that I said ToolTips pop up frequently. Why not always? Or never?
Well if it were never, this would be a silly post. OK, sillier post. But why not always? Two reasons. Outside of the operating system itself, I think it depends on the program. If the programmer included ToolTips, they happen. If she didn't, they don't.
That explains why they either exist or don't for a particular button. But you may notice that, on the Start Button for example, the ToolTip will pop up sometimes and not others. I've never figured out a pattern. Of course there's a limit to how much of my life I'm willing to spend hovering a cursor over a button in Windows.
It could be that once you click on the Start Button a few times Windows assumes you've figured it out and don't need a ToolTip any more. If that's the case, I'd like the ToolTip guy to talk to the Clippy guy.