The answer I learned growing up was that "we don't want to embarrass the bread because we are first making kiddush" or some variation thereof. I later learned that is a zecher for the ma'n which fell with a coating on top and bottom. Where does that first answer come from? And, if it is a legitimate source, what does it really mean (ie bread doesn't have feelings!)?
The Tur (OC 271) here brings three reasons why we cover the challah.
- 1) To establish that the meal is coming because of the kiddush. The Talmud (Pesachim 100b) quotes a braita that says one shouldn't bring the "table" out until after kiddush because, according to the Shi'iltot (#54), we want to show that the kiddush is defining the meal. Tosafot there comments that this only applies in the times of the Talmud where each member at a meal had his own table; now that it is a big hassle to bring out the entire table after kiddush, we cover the challah to pretend it's not there and after kiddush we "bring it out" to the table. Essentially, we want the meal to not start until after kiddush.
- 2) To avoid embarrassment. The Tur quotes a Yerushalmi (which is not found in our editions of the Talmud) that we don't want the bread to be embarrassed; however he qualifies what he means. In general when faced with foods of different brachot, we first say the bracha on the species that comes first in the list of the Seven Species of Israel, so we would usually say the HaMotzi on the bread (wheat) before the HaGafen on wine (grapes). But since we want to say kiddush before eating bread, we have a halakhic dilemma of precedence. So, the Yerushalmi suggests covering the challah to hide it while we drink the wine first. It seems to me that the any literal feeling bad for the bread's feelings is a later, homiletic interpretation of the Yerushalmi's rule. (I note that the earliest I have found this understanding is in the Mordechai Tosefet Pesachim 611, although he doesn't mention it in the name of the Yerushalmi.)
- 3) To commemorate the manna. The midrash (Yoma 75b) says that the manna that fell in the dessert was encased on both sides by a layer of dew. Thus, the Tur suggests (and this is also mentioned in Tosfot Pesachim 100b) that we cover the challah on both sides with a cloth to commemorate the manna. Note that some wonder why we commemorate the manna on shabbat when no manna fell. A possible answer is that since we are using lechem mishna to commemorate the double portion of manna that fell on Friday, we continue the commemoration through the covers. However, not all are impressed with the answer, although the Aruch HaShulchan notes (OC 271:22) that the common practice in eastern Europe was to follow this rule.
Some practical differences emerge from this discussion.
- 1) Bottom layer. As mentioned above, only according to the opinion of the manna does one need a bottom layer.
- 2) Duration. For the embarrassment reason it would seem to be sufficient to cover the challah until after kiddush (or potentially until after Borei Peri HaGafen.) For the tables reason, possible until after one washes (potentially only until after one drinks the wine.) For the manna reason, possibly even until after the blessing of HaMotzi. From the other side, for the table reason, one should make sure to cover the bread as soon as it is put out. For the other two it is sufficient to cover it before kiddush.
- 3) Kiddush on Bread. Only for the manna reason would it make sense to cover the bread if one is making kiddush on bread (ie. if he had no wine).
- 4) Importance. Arguably, according to the tables reason, covering the bread is only a secondary solution and it might be better to not even take out the bread until after kiddush. (The Shi'itot noted above in fact implies that covering is only a bedieved solution.)
- 5) Perhaps the biggest difference regards what needs to be covered. According to the manna reason, only the challah needs to be covered. According to the embarrassment reason, any food whose blessing precedes that of wine (HaMotzi and Mezonot) needs to be covered. According to the tables reason, any food at all that is part of the meal should be covered.