Shabbos, Chapter 23, Mishnah 2. The Mishnah opens with the law of counting one's guests and the courses to be served: "Moneh adam es orchav v'es parparosav mipiv, aval lo min haksav" - a person may count his guests and the courses on his menu by heart on Shabbos, but he may not read them from a written list, neither the guest list nor the details of the menu.
The Gemara offers two reasons for this:
The concern that he may erase one of the guests' names or one of the items on the menu from the list.
The concern that he will become accustomed to reading 'shtarei hedyotos' on Shabbos, that is, documents dealing with business matters and the like. For this reason the Sages forbade even reading a menu or a guest list, lest it lead a person to read business documents.
Casting lots at the table:
The Mishnah continues: "Mefis adam im banav v'im bnei beiso al hashulchan" - a person may cast lots with his children and with the members of his household over the portions set on his table. With others, strangers who are not members of his household, this is forbidden, out of concern that it may lead to measuring the portions, and one does not measure on Shabbos except for the sake of a mitzvah. But with the members of his household, with whom a person is not particular, there is no concern that he will come to measure.
To this the Mishnah adds a qualification: "Uvilvad shelo yiskaven la'asos manah gedolah k'neged ketanah" - when there are large portions and small portions, and the lot determines who will win the large one and who will make do with the small one, this has the character of gambling, and it is forbidden.
The Gemara clarifies how the Mishnah is to be read: with members of the household it is permitted to cast lots even when one portion is large and the other small, since members of a household are not particular with one another about the size of the portions. The qualification forbidding a large portion against a small one was stated specifically with regard to others. It emerges that the Mishnah means: one may cast lots with the members of his household at the table on Shabbos, but not with others.
Casting lots during the week - the law of asmachta:
All of the above applies on Shabbos. During the week it is permitted to cast lots over food even with others, provided that one portion is not large and the other small, since that is a form of gambling, and even during the week it is not permitted. The reason is what is known in the language of the Gemara as asmachta: the gambler does not fully resolve to transfer ownership of what he has staked in the wager, because he assumes that he will be the one to win. There is no complete resolve here and no full intention to hand over the money or the object he staked, and therefore when he loses he is disappointed, since from the outset he never intended to give it up entirely. This is the rule 'asmachta lo kanya' - an acquisition made on the strength of an asmachta does not take effect. For this reason, even during the week one should not do this when one person has a small portion and another a large one.
Casting lots over sacrificial portions on Yom Tov:
The Mishnah concludes: "Umetilin chalashim al hakodashim b'Yom Tov, aval lo al hamanos" - the portions of the offerings may be distributed among the kohanim by lot on Yom Tov, in the case of offerings that were slaughtered on Yom Tov itself, but not the portions of offerings that were slaughtered before Yom Tov. The reason: the lottery for those offerings could have been conducted before Yom Tov, and therefore it is not permitted to conduct it on Yom Tov.
In summary: in this Mishnah we learned three laws: counting guests and portions is permitted by heart and forbidden from a written list, out of concern for erasing and out of concern for reading business documents; casting lots over portions is permitted with members of the household and forbidden with others, while weighing a large portion against a small one is forbidden because of asmachta even during the week; and casting lots over sacrificial portions is permitted on Yom Tov for offerings slaughtered on that day, but not for those slaughtered beforehand.