You know that water freezes at zero degrees Celsius, but have you ever wondered why water freezes? Each water particle exerts a force of attraction towards each other. And that is why water is is liquid: the force of attraction between water particles is strong enough to hold them together, but not strong enough to draw them tightly into a solid. Because the force of attraction amongst the water particles is so weak, if you pour the water on the table, it will run and start to spread all over.
Heat Is Energy
Heat is energy. I’m sure you know that. The more heat a substance acquires, the more energized its particles get. If you place water in a pot and place the pot on a lighted stove, the heat will transfer from the fire to the bottom of the pot and then to the water. As the water gets more and more heated, its particles gain more and more energy. The more energy the water particles gain, the faster they start to move about, until they break free from the force of attraction. When this happens, the water has converted to its gaseous state.
Related: The Water Cycle
So if heat is energy which causes the water particles to move faster and break away from each other into steam, what happens when the water loses heat? As the water gains heat, the particles gain energy and increase speed of movement; but what happens if Superman picks up the pot from the stove seconds before the water starts to convert to gas, and flies away to the North Pole? As Superman approaches the North Pole, where the energy of the sun diminishes, the water will get cooler and cooler. Having no heat to energize the particles, the particles will move slower and slower until their rate of movement becomes so slow and weak that they cluster together.
Energy causes things to move faster; and in reality, what really causes water to freeze is the lack of energy in its particles: making it difficult to move away from each other, and easier for the inter-particulate force of attraction to bind them together.
But this principle is true not only for water. Every substance in the universe loses energy as it gets cooler, and their particles draw closer together the cooler, and less energy they get.