(1) One should not walk bare-headed (the distance of) four cubits1.
(2) Nowadays there is an additional prohibition, not only to walk a short distance, but even to sit in the house bare-headed2.
(3) It should be even worn during sleep at night3.
(4) Small children should also be raised to cover their heads4.
Keepa Ever try to keep one on when you are asleep? It's silly. My rabbi held his in his hand when he took a speed boat ride. It wouldn't stay on, and he didn't want to feed it to the water. And what does that G'mora say, which quotes a man who didn't where one (tiztis), and his rabbi was fine with his reasons. Nothing like giving the whole story when you tell people they have to wear a keepa.
Paraphrasing: "I am not a member of an organized Religion; I'm a Jew."
The comparison between sleeping and a speed boat ride doesn't hold water. Obviously the kippah might fall off while you sleep. The point is to go to sleep with it on. It's not silly. While the long history of kippah is a complex one, the purpose of this article I think was to give a clear explanation of what normative halacha is today.