What is the time range in which one can say the prayer of Shema at night? Why did the mishna start in mid topic? Why with the night prayer and not the morning prayer of Shema? Why did they describe the time based on an action and not in a time frame connected to the sun (from when three stars come out)? The gemara brings several braitot that deal with the same question but bring different answers and the gemara tries to figure out what the different times are and how these different sources work together.