Why does a heavy ship float but a small stone sinks?

A ship made of steel can weigh thousands of tonnes, yet it floats. A small stone weighs almost nothing, yet it sinks. The secret is not weight — it is shape.

When an object goes into water, it pushes some water out of the way. The water pushes back with a force called buoyancy. The more water the object pushes aside, the bigger the push back.

A stone is small and solid, so it pushes aside only a little water. The push back is weaker than the stone’s weight, so it sinks.

A ship is shaped like a huge hollow bowl. Its wide body pushes aside an enormous amount of water. The push back is strong enough to hold up the whole ship — steel, cargo and all. Fill the ship with water, and it loses that hollow space and sinks.