Energy is the ability to do work or make things happen. When you run across a playground, climb stairs, or throw a ball, you're using energy stored in your body. When a car drives down the street, it uses energy from gasoline. When you turn on a light, electrical energy flows through the bulb to create brightness.

Energy comes in many forms. Kinetic energy is the energy of movement: a rolling bowling ball, a spinning wind turbine, or water rushing down a waterfall all have kinetic energy. Potential energy is stored energy waiting to be released: a stretched rubber band, water held behind a dam, or a book sitting on a high shelf all have potential energy that could turn into motion.

Scientists discovered something remarkable: energy can't be created from nothing or destroyed. It just changes form. When you eat food, your body converts chemical energy into the energy you need to think, move, and grow. When you hit a tennis ball with a racket, energy transfers from your moving arm to the ball, sending it flying across the court.

People also use energy to describe personal liveliness or enthusiasm: “She brought great energy to the project” means she worked with excitement and vigor. A high-energy person seems full of life and enthusiasm, while someone with low energy might feel tired or sluggish.