He listened to lectures, read books, tried to take in the solutions and facts presented, but there was a Question that disturbed him again and again: "yes, the solution seems to work, it appears to be correct; but how is it possible to invent such a solution? Yes, this experiment seems to work, this appears to be a fact; but how can people discover such facts? And how could I invent or discover such things by myself?" Today the author is teaching mathematics in a university; he thinks or hopes that some of his more eager students ask similar Questions and he tries to satisfy their curiosity.