Eruvin, Chapter 10, Mishnah 4. Our Mishnah deals with two topics: placing objects on a ledge that juts out over a public domain, and handling objects by a person standing in one domain when the object rests in another.
"Zeez shelifnei chalon - nosen alav v'notel mimenu": - a ledge in front of a window, one may place objects on it and take from it.
A shelf (ledge) that juts out in front of a window over a public domain, at a height of more than ten tefachim, is not considered part of the public domain, since anything above ten tefachim from the public domain has the status of a makom petur (an exempt area). Therefore it is permitted on Shabbos to place objects on it and to take them from it through the window into the private domain.
The Gemara, however, sets a limitation on this permission: one may place on that ledge only breakable vessels - vessels that would shatter if they fell into the public domain. There are two reasons for this:
Concern for carrying out: With any vessel that does not break, there is a concern that if it falls, the person will come to pick it up from the public domain and bring it into the private domain - a Torah prohibition of desecrating Shabbos. With vessels that break, he has no need at all to retrieve them.
Added care: The Rishonim explain that with breakable vessels a person is far more careful and takes pains that they should not fall.
"Omed adam birshus hayachid u'metaltel birshus harabim": - a person may stand in a private domain and handle objects in the public domain.
The Mishnah continues: a person may stand in a private domain and extend his hand into the public domain in order to handle objects there - provided that he does not bring them into the private domain, for such bringing in is forbidden. And so too the reverse: he may stand in the public domain and handle objects resting in the private domain, and we do not suspect that he might bring them into the domain in which he is standing.
The Gemara's limitation - vessels that he needs:
The Gemara explains that this permission was stated specifically regarding vessels that the person has no need for in the domain where he is standing. But a vessel resting in the second domain - whether the public domain or the private domain - which he needs right now in the place where he stands, carries a concern that he might draw it toward himself, and therefore he is not permitted to handle it, even though he is not bringing it into his domain at the moment. With vessels that he does not need, however, he may handle them in the second domain.
"U'vilvad shelo yotzi chutz me'arba amos": - provided that he does not carry it beyond four amos.
The conclusion of the Mishnah adds a condition: even though one standing in a private domain was permitted to handle an object resting in the public domain, he may not move it more than four amos, since moving an object more than four amos in a public domain is a prohibited labor in its own right. Therefore, even though we do not suspect that he will bring the object into the private domain, moving it within the public domain more than four amos is certainly forbidden, even though he himself is standing in the private domain.
In summary: In this Mishnah we learned the law of the ledge in front of the window, which because of its height above ten tefachim has the status of a makom petur, and therefore one may place objects on it and take them from it - and only breakable vessels, out of concern that he might carry from the public domain, and because with such vessels a person is more careful. We also learned that one may stand in one domain and handle objects in the second domain, provided that he does not need them in the place where he is standing, and provided that he does not move them within the public domain more than four amos.