How can a ship made of iron float when an iron block sinks?

An iron ship floats because of its shape, not its material. Water pushes up on anything placed in it with a force equal to the weight of the water it shoves aside. A solid one-ton block of iron is nearly eight times as dense as water, so it only pushes aside an eighth of a ton of water: not enough, and it sinks. Hammer the same ton of iron into a bowl and it pushes aside far more water as it settles. The deeper it sits, the harder the water pushes back, until the push equals one ton and it stops sinking. That is how every ship floats: it settles until it has displaced its own weight of water.

That upward push is called buoyancy, and it comes from pressure. Water gets heavier the deeper you go, because more water is stacked on top. So the bottom of a boat is squeezed harder than its top, and the difference pushes it up.

The rule is named after Archimedes, who wrote it down around 246 BC. Legend says he worked it out in the bath, watched the water rise, shouted 'Eureka!' and ran naked through the streets of Syracuse. The story first appears two centuries later, though, and some scholars doubt it happened that way.