Here's why:
* Parenthesis are used to add extra information or clarify something. The information inside the parenthesis is not essential to the meaning of the sentence.
* The period (or other punctuation mark) goes at the end of the entire sentence, even if the parenthesis contain a complete sentence.
Examples:
* Correct: I went to the store (which is right down the street) to buy milk.
* Incorrect: I went to the store (which is right down the street).
* Correct: The cat (who is named Fluffy) loves to sleep in sunbeams.
* Incorrect: The cat (who is named Fluffy).
However, there are a few exceptions:
* If the parenthetical statement is a complete sentence and you want it to stand alone, you can put a period inside the closing parenthesis. This is usually only done for very specific purposes, like separating quotes.
* If the parenthetical statement is an abbreviation, the period for the abbreviation goes inside the closing parenthesis.
Let me know if you have any other punctuation questions!