Edit: Actually that's generally I guess triple click. Double to select a word.
Having a mental model of "this button needs to be double clicked" gets them the result they want, even if that's not a very accurate reflection of the computer.
So, the rule:
List of files on your computer or desktop? Double-click. Otherwise? Don't.
In practice: adherence to this ranges from perfect to abysmal. And users who don’t understand the computer well may not know how to think about whether a given UI element is selectable or not.
So many people have absolutely no concept of different windows let alone a browser. They run Chrome or IE maximized and that is "the Internet". They'll have tons of tabs open because they don't understand tabs and how to navigate them or that they can be closed.
A problem with billions of people using computers is that only a tiny fraction have working knowledge of them, an even smaller fraction understand them. Most people only understand operations by rote.