Shabbos, Mishnah 1. This Mishnah deals with a person traveling on the road when Shabbos arrives: what should he do with his purse? The concern is that out of worry for his money he will come to do something improper, and in the end he will carry the purse four amos in a public domain.
Handing the purse to a non-Jew:
The Mishnah says: "Mi shehechshich lo baderech" - one upon whom darkness fell while he was on the road on Friday - should give his purse to a non-Jew who is traveling with him. Although ordinarily a non-Jew may not carry an object on behalf of a Jew on Shabbos, here the Sages permitted it, precisely because of that same concern: if this leniency were not available, the person might carry the purse himself. For that reason they permitted handing it to a non-Jew.
Placing the purse on the donkey:
If there is no non-Jew with him, he may place the purse on his donkey. Ordinarily a person may not allow his animal to perform labor for him - this is the prohibition of 'mechamer'. The permission here applies only in a particular manner:
The lifting: The person himself lifts the purse and places it on the donkey while the donkey is walking.
The setting down: Before the donkey comes to a stop, the person takes the purse off of it.
The result is that the donkey never performed both a lifting and a setting down, so no complete labor was done by it, and the person himself likewise did not perform a complete labor. However, this leniency applies only when there is no non-Jew with him; if it is possible, it is preferable that the non-Jew carry the purse.
One who enters the city as Shabbos begins:
Now the Mishnah turns to a different case: not a person delayed on the road, but one who enters the city exactly as Shabbos is beginning, with no time left to unload the animal. The animal may not be left loaded on Shabbos, because of tzaar baalei chaim - the burden causes it suffering. Therefore, when he reaches the outer courtyard, the secure place where the items can be set down, the order is as follows:
Utensils that may be handled: He takes these off first with his hands.
Utensils that may not be handled on Shabbos - muktzeh: He unties the ropes and the bundles fall on their own.
Bundles containing breakable items:
The Gemara adds that if the bundles contain breakable items, a distinction must be made:
Small bundles: He may place cushions underneath so that they will fall onto them. True, ordinarily there is a concern about placing muktzeh on top of a utensil, because of 'mevatel keli meiheichano' - since this effectively fixes the utensil in its place, as was discussed in the third chapter; but here, since the bundles are very small, the cushions can be slipped out from under them immediately after they land, and this is not considered 'mevatel keli meiheichano'. The cushions remain permitted to be handled and are not fixed in place.
Large, breakable bundles: Here the problem does exist, and one may not place the cushions underneath them.
The Rambam writes that in such a situation one should untie the bundles very slowly, in the hope that they will not break. But certainly the items may not be left on the donkey, on the animal, because of tzaar baalei chaim - the suffering caused to it by the load.
In summary: We learned the law of one upon whom darkness fell on the road - he gives his purse to a non-Jew, and in the absence of a non-Jew he places it on the donkey in a manner that involves no complete lifting and setting down. We also learned the order of unloading an animal that enters the city as Shabbos begins: taking off the utensils that may be handled, untying the ropes for the muktzeh utensils, and the distinction between small bundles, under which cushions may be placed, and large breakable bundles - and all of this because of tzaar baalei chaim.