“If opportunity doesn't knock, build a door”
Variant: If opportunity doesn’t knock, build a door.
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Milton Berle 9
American comedian and actor 1908–2002Related quotes

“Opportunity may knock only once but temptation leans on the door bell”
Source: Oprah Winfrey Speaks: Insights from the World's Most Influential Voice

“We're knocking on the door of success.”
5-Oct-2005, DCFC website
Said after 9 games without a win.

“A man knocking on the door of a brothel is looking for God.”
The source is actually a 1945 book by Bruce Marshall, The World, The Flesh, and Father Smith, in which he says, "...the young man who rings the bell at the brothel is unconsciously looking for God."
Misattributed

“They are so busy knocking that they do not notice that the door is open!”
Hercule Poirot’s Early Cases (1974)

On Later With Jools Holland (21 May 2004)
About the Notre Dame fire, Odds & Ends

“Opportunity only knocks once, but temptation leans on the doorbell.”
Attributed quotes