“Ancient Rule of Twenty-one: if you do anything for twenty-one days in a row, it will be installed as a habit.”
Source: The Monk Who Sold His Ferrari: A Remarkable Story About Living Your Dreams
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Robin S. Sharma 52
Canadian self help writer 1965Related quotes

“The most successful people have the same twenty-four hours in a day that you do.”
Source: Disrupt You! (2015), p. 42

At the Paradiso, Amsterdam; April 24, 1995
Stage banter

“Twenty years of schoolin'
And they put you on the day shift”
Song lyrics, Bringing It All Back Home (1965), Subterranean Homesick Blues

As quoted in "Mansfield Park and Film : An Interview with Patricia Rozema" by Hiba Moussa, in Literature/Film Quarterly 32, No. 4 (2004), p. 255
Context: You cannot underestimate what a radical thing it is to change from one art form to another. An author slaves to start with just the right word, phrase, sentence, and paragraph. The sounds of the words are crucial. But all the demands of words and prose are lifted when you make a movie. The physical presence makes many unnecessary and some necessary ones impossible. So you serve two masters as an adapting filmmaker: the author's intention and the needs of film. Sometimes "fidelity" can mean only focusing on one day of a story told over twenty years in a book.

“Sweet childish days, that were as long
As twenty days are now.”
To a Butterfly (I've Watched You Now a Full Half-Hour), st. 2 (1801).

… The twenty-five percent is for error.
Pauling's reply to an audience question about his ethical system, following his lecture circa 1961 at Monterey Peninsula College, in Monterey, California.
1990s