A simplified version of the First Law of Thermodynamics states that energy in a system cannot be created or destroyed. Also called the conservation of energy, this principle indicates that all energy must come from and go somewhere when events occur. Energy can move from one place to another or change from one form to another, but the total amount of energy remains constant.

The exception to the simplified version of the First Law presented above is that matter and energy can be transformed into one another. All matter began as energy, shortly after the big bang. After the big bang, most of the energy in the universe has come from nuclear fusion, in which matter has been transformed back into energy. The transformation of matter into light energy, as four hydrogen atoms fuse into a helium atom inside the Sun, continually produces energy inside of stars. Three other sources of energy in the solar system are:

- The heat that remains from the origin of the solar system. Earth and Venus still have molten cores, but the core of Mars has apparently lost its heat.

- The decay of radioactive atoms that were formed in the supernova that preceded the solar system. Most of the heat of the molten cores of Earth and Venus come from radioactive decay of some of the atoms within them.

- The heat that results from the effects of gravity. Much of the energy that creates volcanic eruptions on the moons of Jupiter results from the gravitational pull of the planet.

Energy comes in several forms. Strictly speaking, thermodynamics deals only with the first two of these forms.

Kinetic energy results from the movement of atoms and molecules. The temperature of matter results from the kinetic energy of its atoms. At absolute zero, there is no kinetic energy. As kinetic energy increases, atoms in its solid state vibrate more and more. When kinetic energy (temperature) increases to the melting point, the atoms or molecules begin to slide past one another, forming a liquid. When kinetic energy increases to the boiling point, the atoms or molecules of a gas move in straight lines until they collide with other matter, or until gravity restrains them.

Potential energy is stored energy that is not currently causing anything to happen. The classic example is a rock at the top of a hill, which is not currently moving but could at any moment roll down the hill. A coiled spring, waiting to expand, and a stretched rubber band, waiting to contract, contain potential energy. Perhaps the most common example is the potential energy that is stored within the bonds of molecules. Some molecules contain a lot of potential energy; some of these molecules are flammable or explosive, under the right circumstances. Potential energy can also be stored by an imbalance of particles, atoms, or molecules. A battery has potential energy; the electrons are not moving, but they can, as soon as the circuit is closed. . .

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