Radioactive materials such as radium on luminous, glow-in-the-dark watch and clock dials might glow, but not people. In order to understand the properties of nuclear radiation and radioactivity, it is necessary to understand the basic structure of matter. An understanding of the basic structure of matter is necessary since nuclear radiation comes from matter, specifically, radioactive matter. A good understanding of the basic composition of an atom is important to understanding the nature and origin of nuclear radiation since nuclear radiation comes from radioactive atoms. An isotope may be unstable due to the change from the stable neutron-to-proton ratio for the element and may be radioactive. The unstable and excited state may promote neutron decay and cause the nucleus to be radioactive and emit nuclear radiation. With all the things covered so far about nuclear radiation, there is another topic to be understood, that is, the damage to living tissue due to radiation-induced ionization of the tissue.