Mass–energy equivalence (E=mc²)

Have you ever felt the warm sun on your face, or run so fast your heart thumped like a drum inside your chest? That's me. I'm the wiggle in a puppy's tail and the zoom in a race car. I am the glow from your night-light that keeps the shadows away and the yummy power in your breakfast that helps you jump, learn, and play all day long. I am in everything that moves, grows, or shines bright. You can't hold me in your hands like a toy, but you can see what I do everywhere you look. I push the clouds across the sky and help the flowers grow toward the sun. I make the music come out of the speakers and the pictures dance on your screen. I am a secret power, all around you, helping things happen. What am I? I am Energy.

For a long, long time, people saw me in all my different disguises, but they didn't know we were all the same person. They saw me as the crackling fire that kept them warm on cold nights, the bright light from the sun that helped their gardens grow, and the mighty push of the wind that sailed their boats across the water. They thought light was just light, motion was just motion, and heat was just heat. They didn't know I was the secret power behind them all. But then, some very curious people, called scientists, started to notice something amazing. One of them was a man named James Prescott Joule, way back in the 1840s. He was very clever and loved doing experiments. He built a machine that used a falling weight to turn a paddle inside a container of water. He discovered that the work of stirring the water made the water get a little bit warmer. It wasn't magic. He realized that motion, which is one form of me, could turn into heat, which is another form of me. It was a huge discovery, like finding a secret key to a mystery. People learned that I don't ever go away or disappear. I just love to change my clothes. I can be the electrical energy sleeping in a wire, then wake up and change into light energy in a lamp, and then become heat energy that warms up the whole room. I am always moving and changing, but I am always there, ready for my next adventure.

Today, you can find me working everywhere you look. I am the electricity that powers your video games and makes the refrigerator keep your juice cold. I am the energy from gasoline that makes cars and buses go vroom-vroom down the street. I am even inside you. The yummy food you eat, like apples and sandwiches, gives your body the energy it needs to think big thoughts, grow taller, and kick a soccer ball all the way across the field. You are using me right now to read these words and understand this story. I am the power that helps scientists launch giant rockets to the stars and helps doctors use special machines to see inside people and help them get better. I am the silent, invisible helper behind almost everything you do. So next time you switch on a light, take a big leap on the playground, or even just take a deep breath, give me a little wave. Remember me. I am Energy, and I am here to help you do amazing things every single day.