Four states of matter are observable in everyday life: solid, liquid, gas, and plasma. Many other states are known to exist only in extreme situations, such as Bose-Einstein condensates, neutron-degenerate matter, and quark-gluon plasma, which only occur in situations of extreme cold, extreme density, and extremely high-energy color charged matter respectively. Some other states are believed to be possible but remain theoretical for now.
A solid has a definite shape and volume because the molecules that make up the solid are packed closely together and move slowly. Solids are often crystalline; examples of crystalline solids include table salt, sugar, diamonds, and many other minerals. Solids are sometimes formed when liquids or gasses are cooled; ice is an example of a cooled liquid that has become solid. Other examples of solids include wood, metal, and rock at room temperature.
A liquid has a definite volume but takes the shape of its container. Examples of liquids include water and oil. Gasses may liquefy when they cool, as is the case with water vapor. This occurs as the molecules in the gas slow down and lose energy. Solids may liquefy when they heat up; molten lava is an example of solid rock which has liquefied as a result of intense heat.
A gas has neither a definite volume nor a definite shape. Some gasses can be seen and felt, while others are intangible for human beings. Examples of gases are air, oxygen, and helium. Earth's atmosphere is made up of gases including nitrogen, oxygen, and carbon dioxide.
Plasma has neither a definite volume nor a definite shape. Plasma often is seen in ionized gases, but it is distinct from gas because it possesses unique properties. Free electrical charges (not bound to atoms or ions) cause the plasma to be electrically conductive. The plasma may be formed by heating and ionizing a gas. Examples of plasma include stars, lightning, fluorescent lights, and neon signs.