Demai, Chapter Three, Mishnah Three
1) If one finds produce on the road and picks it up in order to eat it, and then decides to hide it, he may not hide it unless he has [first] tithed it.
2) But if from the outset he picked it up only in order to guard it against being destroyed, he is exempt [from tithing it].
3) Any produce which one may not sell [in the condition of] demai, he may not send it [as a gift] to his friend [in the condition of] demai.
4) Rabbi Yose permits [one to send as a gift produce] that is certainly untithed, on condition that he makes the matter known to the recipient.
Section one: In this scenario, he is liable to tithe the produce because he picked it up in order to eat it. He cant store or hide the produce away without first tithing it, lest someone come by and find the food and eat it while it is untithed. This is similar to what we learned in section one of mishnah two. Again, a person has to make sure he doesnt cause someone else to unwittingly eat untithed produce.
Section two: However, if he picks the produce up only to prevent it from being destroyed, without having the intention of eating it, he is not liable to tithe it. In this case he can put it back down without tithing it. This is similar to the end of mishnah two.
Section three: We learned in chapter 2, mishnayot 4-5, that it is forbidden to sell certain types of produce while they are in the state of demai. Our mishnah teaches that just as one is not allowed to sell such food, so too one is not allowed to send it as a gift to ones friend. We might have thought that since it is a gift, we do not force him to tithe the produce before he sends it. Our mishnah demonstrates that the reason that he must tithe the produce before he sends it to someone else is lest the recipient it eat while it is untithed. Such a reason exists whether he is selling it or giving it away.
Section four: Rabbi Yose disagrees and allows one to send untithed produce as a gift to ones friend, as long as he informs his friend that the produce needs to be tithed. However, Rabbi Yose allows one to send only produce which has definitely not been tithed. If the produce is only demai (doubtfully tithed), he cant send it without first tithing it, lest the recipient act too leniently with regard to demai. We can see here that the rabbis feared that not everyone would observe the laws of demai, and that a person might act leniently when it comes to demai, even though he generally does tithe. It is of course hard to tell what other Jews, non-rabbis, thought about the entire concept of demai, but one can easily imagine that it may not have made them all that enthusiastic.