Kirchhoff's law of thermal radiation rarefied material such as air, or transparent material, through which observations can be made. The interface is not a material body and can neither emit nor absorb. It is a mathematical surface belonging jointly to the two media that touch it. It is the site of refraction of radiation that penetrates it and of reflection of radiation that does not. As such it obeys the Helmholtz reciprocity principle. The opaque body is considered to have a material interior that absorbs all and scatters or transmits none of the radiation that reaches it through refraction at the interface. In this sense
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