The third paragraph of Shema talks about tzitzis, which are not required at night. Nevertheless, that paragraph is still recited at night because it also talks about the exodus from Egypt. Rabbi Elazar ben Azariah, though young, was as learned as a 70-year-old. He said that he never understood why this paragraph should be recited at night until he heard Ben Zoma’s explanation. The Torah says, “You should remember the day you left Egypt all the days of your life” (Deuteronomy 16:3). According to Ben Zoma, “days” would mean just the days but “all the days” extends the obligation to include the nights. The Sages have a different opinion: they say that “days” means this world but “all the days” extends the obligation to include the Messianic era. (If this sounds familiar, it’s in the Passover Haggadah.)
Let’s say that someone was reading the Torah when the time to recite Shema arrived. If he happened to read those passages at the proper time, would he fulfill his obligation? That all depends on whether or not one had the intention to perform the mitzvah through his reading. Rabbi Meir says that between the paragraphs of Shema, one may greet and respond to a person he honors; in the middle of the paragraphs, he may greet and respond to someone he fears (such as a ruler who could punish him). Rabbi Yehuda says that in the middle of the paragraphs, one may greet out of fear and respond out of respect; between the paragraphs, he may greet out of respect and respond to anyone.