"The obviously weary young woman tremulously entered the room. She had already been to all the “experts” – doctors, wet-nurses – even to Chassidic Rebbes for a brachah. All to no avail. Her infant son, premature and underweight, strangely refused to nurse on Shabbos and Yom Tov. Formula was out of the question for the impoverished family, and the long Succos holiday was approaching. Her neighbors terrified her with old wives’ tales, and well-meaning relatives thrust assorted amulets open her. Nothing seemed to help and this was her last desperate hope.

The imposing-looking rabbi, with the head that seemed too massive for his frail body, looked up from his Gemara and listened patiently to the bizarre problem. Without a moment’s hesitation, he counseled her: “Wear your weekday clothing when you nurse the infant on Shabbos and Yom Tov.” A few days later, the woman’s husband came back beaming: “Rabbi, you’re a prophet, you have ruach hakodesh! Your advice worked!” he exclaimed.

The rabbi seemed amused. “This matter is explicitly recorded in a Tosafos in Bava Kamma 37a” he explained. “The Mishnah states that an ox can be considered a mu’ad (needing extra guarding because he has gored a number of times) on Shabbos, but not on weekdays. Tosafos explains that on Shabbos an ox may not recognize those usually familiar to him because they are wearing different clothing. We see from Tosafos that intelligence at its simplest stages identifies people by outer factors such as color and shape. This poor woman had changed her garment in honor of Shabbos and her particularly sensitive infant mistook the change of clothing as a change of person.”

(http://www.autoadmit.com/thread.php?thread_id=3771755&forum_id=613#34532624)