If opportunity doesn’t knock, build a door.

This Milton Berle quote is a humorous take on the age-old proverb "Opportunity knocks but once." The original proverb dates back to at least the 1500s. Berle's quote has been credited to him in a number of places, including the book Milton Berle’s Private Joke File: Over 10,000 of His Best Gags, Anecdotes, and One-Liners (1989). However, it is not clear when Berle originally spoke or wrote these words; the quote does not appear in his 1993 book, Milton Berle’s Private Joke File: Over 10,000 of His Best Gags, Anecdotes, and One-Liners.  The meaning of the quote is that you should not sit around waiting forever for a great opportunity to find you -- sometimes you have to take initiative to increase your chances of encountering opportunities.