In addition to Houdi's explanation, this is enlightening as well: "The proverb has been traced back to Geoffrey Chaucer's 'Troilus and Criseyde' (1385). George Herbert wrote in 1651: 'Whose house is of glass, must not throw stones at another.' This saying is first cited in the United States in 'William & Mary College Quarterly' (1710). Twenty-six later Benjamin Franklin wrote, 'Don't throw stones at your neighbors', if your own windows are glass.' 'To live in a glass house' is used as a figure of speech referring to vulnerability." [From "Random House Dictionary of Popular Proverbs and Sayings" (1996) by Gregory Y. Titelman (Random House, New York, 1996).]