Letting your animals rest

The fifth and sixth chapters of Shabbat discuss what items may be carried in public on the body of a person or animal, as clothing or as jewelry, and are thus excluded from the prohibition of hotsa'ah.

If you were to ask me how these laws should be organized, I would say that the Mishnah should start by discussing what items may be worn by people, followed by what items may be worn by an animal. But the fifth perek deals with animals, and the sixth perek deals with people. Why is that?

The Yalkut Bi'urim in the Mesivta offers a few answers from the aharonim. I suggest that the Mishnah begins with the more interesting, “haviva leih” topic.

The prohibition of hotsa'ah for animals is unique in its sources and in its application.

Making a keli unusable on Shabbat

The mishnah on Shabbat 42b states:

אין נותנין כלי תחת הנר לקבל בו את השמן, ואם נתנוה מבעוד יום - מותר. ואין ניאותין ממנו, לפי שאינו מן המוכן.

One may not place a keli under a lamp, on Shabbat, in order to catch the oil that may drip.

Rav Hisda says the principle also applies to a placing a keli under a hen to catch a newly hatched egg:

אמר רב חסדא: אף על פי שאמרו אין נותנין כלי תחת תרנגולת לקבל ביצתה - אבל כופה עליה כלי שלא תשבר.

Rav Yosef explains Rav Hisda on 43a:

רב יוסף אמר: היינו טעמא דרב חסדא - משום דקא מבטל כלי מהיכנו.

By causing a new egg, which is muktsah, to rest on the keli, you are mevattel keli me-heikhano, “nullifying the readiness of the keli.”

Abaye raises a number of cases against Rav Yosef that also involve bittul keli me-heikhano at first glance, including supporting a broken beam in the ceiling with a bench, placing a keli under a leaking roof, and using an overturned basket as a step for baby chicks to enter and exit their coop.

What is this prohibition of bittul keli me-heikhano?