Eruvin, Chapter 3, Mishnah 2. This mishnah details the types of food that may be used for eruv techumin, for eruv chatzeros and for shitufei mevo'os. Although some of these foods involve halachic complications, some of them may be used - and some may not.
The foods that are valid for an eruv:
"Me'arvin bi'demai" - one may make an eruv with demai. Demai is produce purchased from an am ha'aretz, where it is uncertain whether tithes were separated from it. Even so, it may be used for an eruv, since the halachah is that one may feed demai to the poor. Since any person can declare his property ownerless and thereby become poor, at which point he would be permitted to eat demai, even a wealthy man may make an eruv with demai.
"U'v'ma'aser rishon shenitlah terumaso" - and with first tithe whose terumah has been taken. First tithe is given to the Levi, and terumas ma'aser (a tenth of the tithe, given to the kohen) has already been separated from it. However, the terumah gedolah - one part in fifty, which the owner was required to separate for the kohen - has not been separated from it. Even so it is valid for an eruv, because the case is one in which it never became obligated in terumah gedolah at all: the tithe was taken while the grain was still in the stalks, before the pile was smoothed over (before the heap of grain was leveled and smoothed). When the first tithe is taken at this stage, the tithe is not obligated in terumah gedolah.
"U'v'ma'aser sheni v'hekdesh shenifdu" - and with second tithe and consecrated property that were redeemed. This refers to second tithe, which is meant to be brought up to Jerusalem, and to hekdesh belonging to the Beis HaMikdash, both of which were redeemed. What is the novelty here? That even if he did not add the fifth - the additional fifth required at redemption (roughly a quarter of the principal) - the redemption is valid, since the fifth does not hold up the redemption of second tithe and hekdesh.
"V'hakohanim b'challah u'v'terumah" - and kohanim may make an eruv with challah and with Terumah. Kohanim may use the challah separated from the dough and the Terumah given to them, since these foods are permitted to them for eating.
The foods that are invalid for an eruv:
"Aval lo b'tevel" - but not with tevel. Tevel is produce from which not all the terumos and tithes have been separated. One may not make an eruv with it, and this is true even if it is only tevel by rabbinic law - for example, produce that grew in a flowerpot rather than in the ground, which is obligated in terumos and tithes only rabbinically.
"V'lo b'ma'aser rishon shelo nitlah terumaso" - and not with first tithe whose terumah has not been taken. Here the meaning is that the terumah gedolah was not separated in a situation where the grain had already become obligated in it: the first tithe was taken and given to the Levi after the pile was smoothed over, after the heap of grain was leveled, at which point it had already become obligated in terumah gedolah. Therefore, even though the first tithe is already in the hands of the Levi, he must separate terumah gedolah from it; and as long as he has not separated it, one may not use it for an eruv, neither for eruv techumin nor for eruv chatzeros.
"V'lo b'ma'aser sheni v'hekdesh shelo nifdu" - and not with second tithe and consecrated property that were not redeemed. The meaning is not that they were not redeemed at all, but that the redemption was performed improperly: second tithe that was redeemed with a piece of silver that does not bear the form of a coin, and hekdesh that was redeemed with land and real estate - a redemption that is not valid.
Sending one's eruv with a messenger:
The mishnah concludes: "HaShole'ach eruvo b'yad cheresh shoteh v'katan, o b'yad mi she'eino modeh b'eruv - eino eruv" - one who sends his eruv by the hand of a deaf-mute, a mentally incompetent person or a minor, or by the hand of one who does not accept the concept of an eruv, it is not an eruv. A person who wishes to place his eruv in a particular spot, so that it will serve as his new center point on Shabbos from which he measures two thousand amos in every direction, may not send the eruv with a deaf-mute, a mentally incompetent person or a minor, and likewise not with an adult of sound mind who does not believe in the idea of an eruv. One may rely neither on a person who is not responsible nor on one who does not accept the eruv to place the eruv in its proper spot.
"V'im amar l'acher l'kablo mimenu - harei zeh eruv" - and if he told another person to receive it from him, it is a valid eruv. If he appointed another person to receive the eruv from them and to place it in its spot, the eruv is valid. However, the Gemara clarifies that this holds only when he saw with his own eyes that the disqualified messenger actually handed the eruv over to the person appointed to receive it from him. Once he has seen that the eruv was handed over to the one appointed to place it in its proper spot, he may rely on the fact that the appointee carried out his mission properly.
In summary: In this mishnah we learned which foods are valid for an eruv - demai, first tithe whose terumah has been taken, second tithe and hekdesh that were redeemed, and challah and Terumah in the case of kohanim - and which are invalid for an eruv: tevel (even rabbinic tevel), first tithe from which terumah gedolah was not separated after it became obligated in it, and second tithe and hekdesh whose redemption was performed improperly. We also learned that one may not send the eruv with a deaf-mute, a mentally incompetent person or a minor, or with one who does not accept the eruv, unless he appointed a qualified person to receive it from them and place it in its spot.