Archimedes' Principle

Have you ever jumped into a swimming pool and suddenly felt as light as a feather. Or maybe you've pushed a beach ball under the water and felt it push right back up, desperate to pop to the surface. That push, that gentle lift you feel in the water, that’s me. I’m a secret helper, an invisible force in every puddle, bathtub, and ocean on Earth. I am the reason a tiny rubber duck bobs happily on the waves and a huge log can float down a river. But it’s a bit strange, isn't it. How can a gigantic cruise ship, made of heavy metal, float so easily, while a little pebble you toss in the water sinks straight to the bottom. Can you imagine a world where no one understood this mystery. For a very long time, people saw me at work every day, but they didn't have a name for me or a reason for my magic. They just knew that some things floated and some things sank. It took a very clever man and a very tricky problem to finally introduce me to the world. I was waiting in the water, ready to be discovered.

My big moment came a very long time ago, in the 3rd century BCE, in a sunny, seaside city in Sicily called Syracuse. A brilliant thinker named Archimedes lived there, and he was known for being able to solve the most difficult puzzles. The king, Hiero II, had a brand new crown made of pure gold, or so he thought. He had a nagging feeling that the goldsmith had cheated him by mixing in some cheaper, lighter silver. The king commanded Archimedes to find out the truth, but there was one giant rule: he could not damage the beautiful crown in any way. Archimedes was stumped. He couldn't melt the crown down or scratch it. He walked around for days, thinking and thinking, but the answer just wouldn't come. One afternoon, feeling tired, he decided to take a bath. As he lowered himself into the full tub, he watched as a large amount of water sloshed over the sides and onto the floor. In that very instant, his eyes widened with an idea. He noticed two things. First, the amount of water that spilled out was the same size as the part of his body he put into the tub. Second, he felt me. He felt that upward push from the water, making him feel lighter. A huge smile spread across his face. He realized that the upward push he felt was exactly as strong as the weight of the water he had pushed aside. He had found it. He was so excited that he leaped out of the bathtub and, forgetting his clothes, ran through the streets of Syracuse shouting, 'Eureka. Eureka.' which means 'I have found it.' in Greek. In that splashy, brilliant moment, I was finally understood. I am Archimedes' Principle.

So how did a bath solve a problem about a crown. Well, Archimedes took the king’s crown and a lump of pure gold that weighed exactly the same. He dunked the lump of pure gold into a bowl of water and measured precisely how much water spilled out. Then, he did the same thing with the crown. And guess what. The crown made more water spill out. Because silver is less dense than gold, it takes up more space for the same weight. By mixing in silver, the cheating goldsmith had made the crown slightly bigger than a pure gold crown would have been. The king had his answer, and I became famous. Since that day, I have been helping people in amazing ways. I am the reason gigantic steel cargo ships, which weigh many thousands of tons, can float peacefully on the ocean. Their hollow, wide shape pushes aside, or displaces, a huge amount of water, and the upward push from that water is strong enough to hold the entire ship up. I am how submarines can dive deep and then rise back to the surface. I am even how hot-air balloons float in the sky, because they push aside the 'sea' of air around them. It all started with one person paying attention to a simple splash in a bathtub. So always keep your eyes open and stay curious, because the next world-changing 'Eureka.' moment could be waiting for you anywhere.