In physics, energy is the capacity to do work. Think of it as the currency for making things happen. To push a box across the floor, to heat a pot of water, or to light up a room, you need to spend some energy. Without energy, everything in the universe would be static and unchanging.

Energy isn't just one thing; it comes in many different forms. These forms describe how the energy is stored or expressed. Let's look at a few of the most common types.

A rolling bowling ball has kinetic energy. The faster it rolls, and the heavier it is, the more kinetic energy it has. A planet orbiting the sun and a tiny electron buzzing around an atom's nucleus both possess kinetic energy.

Imagine a book sitting on a high shelf. It's not moving, but it has the potential to create change. If it falls, its stored energy (gravitational potential energy) will be converted into the energy of motion. Similarly, the chemical bonds in a matchstick hold chemical potential energy, which is released as heat and light when struck.

Two other important forms are thermal and chemical energy. Thermal energy is related to the temperature of an object and comes from the tiny, random motions of its atoms and molecules. A cup of hot coffee has more thermal energy than a cup of iced tea.

Chemical energy is stored in the bonds that hold atoms together to form molecules. The food we eat, the wood in a fireplace, and the gasoline in a car all contain chemical energy, which can be released through chemical reactions.

A fundamental rule governs all these forms of energy. It's called the principle of energy conservation, and it's one of the most important laws in all of physics. It states that energy cannot be created or destroyed. It can only change from one form to another.

Let's go back to the book on the shelf. As it sits there, it has potential energy. When it falls, that potential energy is transformed into kinetic energy. Just before it hits the floor, nearly all its initial potential energy has become kinetic energy. When it hits the ground, that kinetic energy doesn't just vanish. It's converted into other forms, primarily thermal energy (the book and floor get slightly warmer) and sound energy.

This principle is always at play. When you eat food, your body converts its chemical energy into the kinetic energy of your muscles and the thermal energy that keeps you warm. An electric power plant doesn't create energy; it transforms chemical or nuclear energy into thermal energy, which then becomes the kinetic energy of a turbine, and finally, electrical energy. The total amount of energy in a closed system always remains the same.