Answer:
Constant temperature variations
Step-by-step explanation:
With temperature constantly heating many molecules within many forms of matter tend to move around. Take air for instance and density. When air is hot the molecules are moving very quickly because they are being what is known as energized (I don't know the scientific word I apologize.) However, when air is cold it tends to get harder to breath and that is because the air is more dense, since the molecules are atoms are much more compacted.
To further detail the process of cooling and heating up matter, water when it is cooled it tends to become ice; a solid based structure. Therefore, making it more dense or more compacted together. However, once heated the molecules begin to spread apart, in which the density lessens and the molecules become a liquid state or a gas state (depending on the amount of heat applied in terms of kelvin.)