When you put something in water, the water pushes up on it. The bigger the object, the more water it shoves out of the way — and the more the water pushes back. We call that push buoyancy.
An old Greek named Archimedes discovered this in his bath. He was so excited he ran out shouting "Eureka!" — Greek for "I found it!"
Imagine a heavy steel ball. It pushes aside only a little water. The push back is small. Down it goes. Now imagine a giant steel ship. It's hollow inside — full of air — so even though it's heavy, it pushes aside tons of water. The push back is huge. It floats.
Try each item — does it float, or sink?
Ships are hollow shells. The shape pushes aside more water than the ship weighs. Float.
Hot-air balloons work the same way, but with air. Hot air is lighter than cool air. The balloon pushes aside more cool air than the whole balloon weighs. Up it goes.
You floating in the sea — your body, full of air-filled lungs, pushes aside enough water to hold you. Salty water (Dead Sea) holds you even better, because salty water weighs more.
Submarines have big tanks inside. To dive, they let water in — heavier, they sink. To rise, they push water out and let air in — lighter, they rise. Same boat, two states. Whales do almost the same thing with their lungs.
A penny weighs less than a ship. The penny sinks. The ship floats. Why? The ship pushes aside way more water for its weight. Shape matters.
Make a ball of clay sink, then flatten it into a boat — same clay, but now it floats. Eureka!