The document discusses the three common states of matter - solid, liquid, and gas. It defines each state and explains that a substance's state depends on whether it is dense and rigid (solid), fluid and takes the shape of its container (liquid), or has no definite shape or volume (gas). It also explains that applying physical forces like temperature and pressure changes can change a substance's state by adding or removing energy, but does not change its chemical properties. Specifically, it notes that water changes from solid to liquid to gas as temperature increases through freezing, melting, and boiling.