You've probably had days when you felt tired. You may even have said, "I don't have much energy today." But what exactly is energy? Simply defined, energy is the ability to do work. Energy can be categorized as either potential energy or kinetic energy. Potential energy is stored energy, like the energy stored in a battery, or the energy of position, like a boulder about to roll down a mountain. Kinetic energy is the energy of motion. To understand how potential and kinetic energy are related, consider a car. The gasoline in the car's tank has potential energy. For the car to move, the car's engine transforms the potential energy of the gasoline to kinetic energy: the motion of the car. What are some other examples of motion in everyday life? Here are a few: We walk or run from place to place. Water in a river flows downstream. Sound waves travel to our ears. All these examples of motion are examples of kinetic energy. Kinetic energy can occur on a large scale, like a jet lifting into the air. Kinetic energy can also occur on a microscopic scale, like the movement of molecules in a glass of water. There are different forms of kinetic energy. One form of kinetic energy is radiant energy, also called light energy. Radiant energy is energy that travels in waves of light. Some of this light we see, like the light that travels from the sun to Earth. Other forms of radiant energy, like radio waves, are invisible to human eyes. Another form of kinetic energy is thermal energy. Thermal energy is also known as heat. Heat is produced when molecules vibrate and collide with each other or with the sides of a container. Heat is often a by-product of motion. For example, think about a car moving along a road. The motion of the car tires along the surface of the road produces heat that can be felt in the tires. A third form of kinetic energy is electrical energy. Electrical energy comes from the movement of charged particles. Electrical energy can be produced in nature, like lightning strikes during a thunderstorm. Humans can also produce electrical energy, using machines. We use electricity in our homes, where wires carry electrical charges to lightbulbs and appliances. Electrical energy can be transformed into other forms of energy, too. For example, when you flip a light switch, electrical energy is transformed into radiant energy that lights the room. After a short time, the lightbulb becomes very hot as the vibrating molecules in the bulb result in thermal energy. A fourth form of kinetic energy is sound energy. Like radiant energy, sound energy also travels in waves that consist of particles in motion. When a sound is produced, the waves travel through air (or another type of matter) until they reach our ears. Our ears convert the motion of the sound waves into electrical impulses that then pass on to our brain. The nature of the sounds we hear depends on the characteristics of the waves. Some sounds, like very high-pitched whistles, are beyond the range of human hearing. When you consider all these different forms of kinetic energy, one fact becomes clear: Our lives are full of motion!