i have to present (explain this chapter to the proffessor) thsi topic in class and my strict proffessor wont appriciate missing any point/topic whatsoever. help me study this chapter pls. like how do i start. make it easier to learn. tell me key points so i dont miss the main idea. my strict proffersor loves asking why on every topic, he may again ask why about the given answer. so include or ask and answer every division (like after 18.1 a set of 10-15 difficult and twisted and connected why questions, again the same for 18.2,18.3,18.4,18.5,18.6,18.7 and so on) also in the end give me links of yt videos that help explain physiology and biochemical aspects of digestive system