Why don't electrons fall into the nucleus?

Why don't electrons fall into the nucleus? I've found three different explanations, so I can't decide what is the truth. One says there is some special "shield" around the nucleus. Other says the electrons can't go to a little place because of their own negative charge, which is a pushing force between the electrons. Other, again, says they can' fall there because of the centrifugal force. Please write only if you know exactly the truth.