Mass–energy equivalence (E=mc²)

I am the warmth you feel from the big, bright sun on a summer afternoon. I am the brilliant light that chases away the shadows in your room when you flip a switch. I am even the secret power that helps a cheetah run faster than the wind. Can you feel me? I hide inside the food you eat, like apples and sandwiches, giving your body the strength to jump high, play hard, and learn new things in school. I may be invisible, but you can see my work everywhere you look. I push cars down the road, make your favorite songs blast out of speakers, and help tiny seeds sprout and grow into tall, strong trees. I am the loud, crackling zap of lightning during a thunderstorm and the quiet, steady hum of your refrigerator keeping your milk cold. For thousands of years, you have seen what I can do, felt my presence, and used my power without even knowing my name. But I think it’s time we were properly introduced. Hello. I am Energy.

For a long, long time, people knew me, but they didn’t quite know what to call me. They felt my warmth while sitting around a crackling campfire, and they used my pushing strength in flowing rivers to turn giant wooden wheels that ground their grain into flour. They were slowly getting to know my many disguises, but they didn't realize I was the same force behind it all. One of my biggest and most wonderful secrets is that I never, ever disappear; I just love to change my clothes. The chemical energy stored in your yummy breakfast becomes the motion energy you use to pedal your bike down the street. That motion also creates a little bit of heat energy that makes you feel warm as you ride. See? I just changed my outfit. A very curious man named Benjamin Franklin wondered if the lightning in the sky was a type of me. On a stormy day around June 10th, 1752, he bravely flew a kite with a key tied to it and proved that lightning was indeed electrical energy. Don’t try that at home. Later on, clever inventors like James Watt figured out how to use my heat energy from steam to power huge engines, which made trains and factories move, changing the world forever. Then, a super-smart scientist with wild hair named Albert Einstein came along. In 1905, he wrote down a very famous but very short recipe: E=mc². This little recipe explained my biggest secret: that everything, even the tiniest speck of dust, is filled with a huge amount of me, just waiting to be released. People were finally learning my language and understanding my rules.

Today, you and I are partners in almost everything you do. I light up your phone screen so you can watch videos, I heat your oven to cook your dinner, and I power the important machines in hospitals that help keep people healthy and safe. I can be light, heat, motion, electricity, sound, and so much more. People have become very clever at asking me to change from one form to another to do all sorts of amazing things. But being good partners means we both have to do our part. Some ways of using me, like burning coal or oil, can make our beautiful Earth a little messy and sick. That's why so many brilliant people are now finding clean and safe ways to work with me. They are building amazing machines to capture my power from the bright sun, the strong wind, and the mighty oceans. Your curiosity is the most important key to our future together. As you grow up, you will discover even more of my secrets and invent new ways for us to build a brighter, cleaner, and more wonderful world. I am all around you, ready to help. I can’t wait to see what amazing things we'll do next.