I have 2 questions about the sugya of eating and drinking simulataneously on 14a:
1) The Gemoro is concerned about food or drink of Teruma becoming Tomei inside the mouth. Wouldn't this be Tumas Beis Hasesorim (touching within an enclosed area)?
2) The Gemoro (+Rashi) first describes a case of someone eating tamei food who puts teruma liquid in his mouth, this is common because people often drink while eating.
The Gemoro then describes a case of someone drinking tamei liquid who puts teruma food in this mouth, which is less common.
Why does the second case have to be drinking tamei liquid first? Couldn't he already have teruma food in his mouth, and we want to discourage him from putting tamei liquid in afterwards?! (The case would then be just as common, but why couldn't we describe this case?].
Meir Eliezer Bergman
1. The Gemara in Kidushin (25a) cites Ula who says that everyone agrees that as far as Tum'ah is concerned, the tongue is considered a revealed place and is not Beis ha'Sesarim. The Rambam (Hilchos Tum'as Meis 1:3) adds that it appears to him that if someone touched Tum'ah with his teeth he also becomes Tamei; since the teeth are connected to the body, they have the same Din as the body. Even though the conclusion of the Gemara in Nidah (42b) is that the throat, where one swallows food, is considered Beis ha'Sesarim, one sees that there are many places in the mouth which do not have the Din of Beis ha'Sesarim.
2. Baruch she'Kivanta -- your question is asked by the Rashba.
(a) The Rashba answers that there is a difference between the type of person who eats Tamei food and the type of person who eats Terumah food. A person who eats Tamei food is generally not so careful about what he eats and, consequently, we are worried that he might drink Terumah liquids at the same time that he is eating the Tamei foods. In contrast, one who eats pure Terumah is presumably a more careful kind of individual, and thus there is no concern that he will forget and make the Terumah become Tamei.
(b) The Rashba gives an alternative answer, according to which the distinction is not between different types of people who are eating the food but rather between the state of the food when it is rendered Tamei. In the case in which one ate Terumah first, this means that the food has already been chewed up and is in a repulsive form. The prohibition against making such food Tamei is not so severe; since it is not fully edible now, Chazal did not view it as sufficiently important to make a Gezeirah in case this might happen. In contrast, if he has eaten Tamei food, it is necessary to make a Gezeirah so that he should not then put Terumah food -- which is perfectly fit for consumption -- into his mouth and thereby render it Tamei and transgress the Mitzvah to keep Terumah food from becoming Tamei.