Maasrot, Chapter Four, Mishnah Two
The entrance of Shabbat makes produce liable for tithes. Since Shabbat is important, any eating done on Shabbat is also significant and cannot be considered chance eating which is exempt from tithes.
Our mishnah contains several issues related to this general rule.
1) Children who have hidden figs [in the field] for Shabbat and they forgot to tithe them, they must not be eaten after Shabbat until they have been tithed.
2) In the case of a basket of fruits for Shabbat:
a) Bet Shammai exempt it from tithes;
b) But Bet Hillel makes it liable.
3) Rabbi Judah says: even one who has gathered a basket of fruit to send as a present to his friend, must not eat of them, until they have been tithed.
Section one: The children hid the figs with the intention of eating them on Shabbat. The mishnah teaches that the figs that they set aside in order to eat on Shabbat have now become liable for tithes and cannot be eaten, even after Shabbat, without first being tithed. The important principle here is that intending to eat something on Shabbat makes it liable for tithes even if the person who had the intention was only a child. [I find this mishnah kind of cute, kids hiding figs to eat on Shabbatreminds me of hiding candy from the kids so that they wont eat it on Shabbat].
Section two: This basket was set aside so that the fruit would be eaten on Shabbat. Bet Shammai holds that setting the basket aside in order for it to be eaten on Shabbat does not make the fruit liable for tithes. As long as the fruit has not yet been brought into a place that makes it liable for tithes, one can continue to eat from it without tithing. Bet Hillel accords greater power to his intent and makes him liable for tithes as soon as he sets it aside for Shabbat.
Section three: Rabbi Judah says that gathering fruit together into a basket in order to send it to a friend makes it liable for tithes, even if the basket was not meant for Shabbat. The idea is that making the food into a gift gives it importance, just as Shabbat does. Therefore, he cannot eat until he tithes.