This is one of the major questions which is asked by the students. Whenever we read mathematics or physics book(generally), we see that many problems are given at the end of the chapter. These eventually test our understanding and application of concepts. I have few questions here. Who creates these problems? Is it the author of the text-book or is it copied from some of the other textbooks? I want to ask these because reading and grasping some derivations or concepts take time. After that, when we solve the problems( mainly the twisted ones) on our own, we may think in different angles and we may apply different manipulations. Some can be solved in two minutes with different thinking and others may require larger time(For example,I have taken three hours for solving one!!).Even when we can't solve it, we may refer to other resources(in extreme cases, even solutions)!!!! Those who have very high intelligence may solve all of them. What I want to say is that solving tough problems itself takes time. How could he(say the author) invent it? Inventing a problem and solving it may take tremendous amount of time. The inventor of these problems may also refer it to his colleagues who may cross-check it. This may also involve some time. So please explain me the process of writing the text-book and also creating the problems. I am a bit curious.