Demai, Chapter 1, Mishnah 4. In the previous mishnah we learned that demai may be fed to the poor and to travelers, that Rabban Gamliel would feed demai to his workers, and that a charity collector who collects from a household that tithes and from a household that does not tithe gives to this one and to that one without concern. Now the mishnah lists a series of uses and actions that may be performed with demai, whereas with certain untithed produce (tevel) there is no permission to use it for these purposes.
"HaDemai me'arvin bo umishtatfin bo":
One may make an eiruv with food that is demai. Which eiruv is being discussed?
Eiruv techumin: A person who wishes to walk more than two thousand cubits from his home or his city on Shabbos places food at a certain spot that represents his place of rest, so that his central point for Shabbos is there.
Eiruv chatzeiros: Several houses that open into a single courtyard and whose residents wish to carry in the courtyard make an eiruv that expresses the food they share together.
The reason for the permission: as we will learn further on, a poor person is permitted to eat demai, and since it is food fit for the use of a poor person, even a wealthy person may use it for his own eiruv chatzeiros.
"Umishtatfin bo" - a similar idea: shituf mevo'os. In the case of an alleyway onto which several courtyards open, the various courtyards need to form a partnership of the alleyway together, similar to eiruv chatzeiros, and demai may be used for this as well.
"Umevarchin alav umezamnin alav":
"Umevarchin alav" - the Rambam explains that this refers to Bircas HaMazon, that one may recite it over eating demai. The Rash and others explain that it refers to the cup of wine over which a berachah is recited, that it may be made from demai.
"Umezamnin alav" - the berachah of zimun that precedes Bircas HaMazon is recited even over eating demai.
"Umafrishin oso arom":
One may separate the tithes of demai even when a person is not wearing clothing. With certain tevel there is a problem with this, since the separation requires a berachah: a mitzvah can be fulfilled even without clothing, but berachos may not be recited without wearing clothing, and therefore one who separates from certain tevel cannot do so unclothed. With demai, however, where there is nothing more than a doubt as to whether the obligation to separate applies at all, the Sages did not institute a berachah, and since there is no berachah, one may separate even without clothing.
"Bein hashemashos":
Bein hashemashos of Shabbos eve is a twilight period that is not certainly Shabbos but rather a doubt, and during this time one may separate the tithes of demai, but one may not separate from certain tevel. This matter is learned in the second chapter of Maseches Shabbos - "Bameh Madlikin" - which many have the practice of reciting after accepting Shabbos.
"Im hikdim ma'aser sheni larishon - ein bechach kelum":
The reading in the language of the mishnah is "im hikdim," meaning that one who separates the second tithe before the first tithe - contrary to the proper order, in which one first separates ten percent for the first tithe and then, from what remains, ten percent for the second tithe - with demai this is of no consequence and there is no need for concern.
What is the concern with certain tevel? Separating the second tithe before the first prevents a person from reciting "vidui ma'asros" - the declaration in Parashas Ki Savo, recited in the fourth year and in the seventh year, which a person can recite only if he fulfilled the mitzvos properly. With certain tevel, one who separated in the reverse order can no longer recite vidui ma'asros; but with demai this does not disqualify.
The oil of a weaver and of a wool comber:
"Shemen shehagardi sach bo es etzbe'osav" - the oil with which the weaver anoints his fingers so that his work will be done properly is subject to demai, because "anointing is like drinking" - anointing is comparable to drinking, and this is derived from the verse.
"Veshel sorek hatzemer" - the oil that the wool comber puts into the wool is exempt from demai, since its purpose is neither for human consumption nor for anointing.
Some discuss the following question: how can it be that oil with which the weaver anoints his fingers is subject to demai, while in the previous mishnah Beis Hillel said that fragrant oil, which is used for anointing, is exempt from demai? Whoever wishes to understand the answer to this question should look at the Tiferes Yisrael, who distinguishes between these two types of oil.
In summary: in this mishnah we enumerated the permitted uses of demai - eiruv and shituf, berachah and zimun, separating unclothed and at bein hashemashos, and separating the second tithe before the first - as well as the distinction between the weaver's oil, which is subject to demai because anointing is like drinking, and the oil of the wool comber, which is exempt. The principle that emerges from all of these is that the Sages were lenient with demai in places where the leniency is needed, because most ordinary people do tithe their produce, and the concern that it was not tithed is a remote concern and a weak doubt.
It is worth remembering this rule, for it will help in understanding many halachos that will come before us further on in Maseches Demai.