It is known that Fermi usually started the investigation of a difficult problem with the question: "What plays the part of a hydrogen atom for this problem?" As for nuclear physics, the answer to this question causes no doubts: its hydrogen atom is the deuteron. It is surprising how many nontrivial problems related to the deuteron can be solved by means of sufficiently simple, sometimes truly back-of-the-envelope analytical calculations. Some of them are considered in this chapter.