Everything is made of matter, which has mass and volume. Matter exists in three states - solid, liquid, and gas. It can be natural or man-made. Atoms are the smallest particles of matter and molecules are formed when two or more atoms join together. Materials have different properties depending on how hard, soft, flexible, or breakable they are. Thermal conductors allow heat to pass through easily while insulators do not. Mixtures are two substances mixed together, either heterogeneously where the separate substances can be seen or homogeneously as a solution where the substances mix at a molecular level. Changes in matter can be physical, involving changes in shape, place or state, or chemical, forming new molecules