When a man and a woman fall in love so that they really belong to each other, the physical side of the relation is this: both of them feel at intervals a peculiar thrill or glow, particularly in the sexual organs, and it naturally culminates after they have gone to bed at night. The man's special sex organ or penis, becomes enlarged and stiffened, instead of soft and limp as ordinarily, and thus it easily enters the passage in the woman's body called the vagina or birth-canal, which leads to the uterus or womb, which as perhaps you already know is the sac in which the egg or embryo grows into a baby. The penis and the vagina are about the same size, as Nature intended them to fit each other. By a rhythmic movement of the penis in and out, the sex act reaches an exciting climax or orgasm, when there is for the woman a peculiarly satisfying contraction of the muscles of the passage and for the man, the expulsion of the semen, the liquid which contains the germs of life. This is followed by a sensation of peaceful happiness and sleepy relaxation. It is the very greatest physical pleasure to be had in all human experience, and it helps very much to increase all other kinds of pleasure also. It is at this time that married people not only are closest to each other physically, but they feel closer to each other in every other way too. It is then most of all that they feel _sure_ they belong to each other. The sex act is called by various names, such as coitus, coition, copulation, cohabitation, sex-intercourse, the sex-embrace, etc. But all these terms refer to the same thing. The first coitus is apt to be somewhat painful for the woman, as there is usually a thin membrane, called the hymen, partly closing the vagina which has to be broken through, but all women do not have it and it varies in size and thickness with different people.