To be a scientific theory, a theory must be tested a large number of times, by different scientists in different places, and must pass the test every time. The theory must be stated exactly, sometimes using mathematics. And it must fit in with all of the other scientific theories. Science has many branches. Physics, chemistry, biology, geology, and astronomy are some of the major branches of science. A scientific theory in one branch of science must hold true in all of the other branches of science. For example, the atomic theory of matter, that all matter is made up of atoms, was discovered using physics, but the chemicals used in chemistry, the living tissue used in biology, the rocks studied in geology, and the planets studied in astronomy are all made up of atoms. The atomic theory of matter holds in every area of science.