Radioactivity is defined as the process by which atoms emit particles and high energy rays from their nuclei. Although most elements can be rendered radioactive, the process occurs naturally in only a very few. The modern understanding of radioactivity is that certain elements release radiation as they decay. Such radiation can be recognized in any of three forms: alpha rays, which are positively charged and relatively weak (clothing, or a few sheets of paper, will stop them); beta rays, which can be either positive or negative and are a bit stronger (half an inch of wood will stop them); and gamma rays, which bear no electrical charge yet are by far the strongest. An element which emits alpha or beta rays transforms as it does so, changing into other elements as it releases energy. For example, uranium becomes thorium when it emits alpha particles. Scientists often create "new...