From Little Town on the Prairie by Laura Ingalls Wilder, written to her by her mother, Caroline Lake Quiner Ingalls, November 15, 1881.
If wisdom's ways you wisely seek,
Five things observe with care:
Of whom you speak,
To whom you speak,
And why, and when, and where.
Or, in the words of CJ Kennedy, "Never miss an opportunity to keep your mouth shut."