Sound amplification by stimulated emission of radiation the axes of the mirrors, it is possible for photons to repeat multiple reflections and force more and more photons to follow them producing an avalanche effect. The number of photons of this coherent laser beam increases and competes the number of photons perished due to losses. The basic necessary condition for the generation of a laser radiation is the population inversion, which can be achieved either by exciting atoms and inducing percussion or by external radiation absorption. A SASER device mimics this procedure using a source-pump to induce a sound beam of phonons. This sound beam propagates not in
A Sound Like Someone Trying Not to Make a Sound make a sound". The child wakes his father and describes the sound to him. The child believes that the sound comes from a "monster with no arms and legs", which "slides on its fur" and "pulls itself along on its teeth". The father and child discover that the sound is coming from the mice in the walls, and the child is comforted by his parent. The two return to sleep. A "Publishers Weekly" reviewer wrote that "In showing young readers that the things that go bump in the night are, in reality, not so scary, Irving succeeds in helping them