1. Every sentence must end with a period, question mark, or exclamation point.

      You don’t really need an example of that either, do you?
    2. Only use periods when they are needed.

      1. Use a period to end a sentence that is a statement or request.

        At first Ford had formed a theory to account for this strange behaviour. If human beings don't keep exercising their lips, he thought, their mouths probably seize up. After a few months' consideration and observation he abandoned this theory in favour of a new one. If they don't keep on exercising their lips, he thought, their brains start working.
      2. Use a period after abbreviations, and after each letter in initials.

        Mr., Mrs.
        F.B.I., C.I.A., U.S.A.
        1. If the abbreviation is an acronym (a set of initials that are pronounced as a word, such as NASA) you do not need a period.

        2. If the abbreviation or initial comes at the end of a sentence, use only one period.