A child in a life vest watches a giant grey steel cargo ship float. A clear underwater cutaway shows a small steel bolt well below the surface, with blue water and a trail of bubbles above it.

Why does a steel ship float?

Ships are made of heavy steel — drop a little steel bolt and it sinks straight to the bottom. So how does a whole steel ship stay on top?

First clue: shove anything down into water and the water shoves back UP on it. Ever tried holding a beach ball under? You can feel it fight to rise.

A chunky grey steel block is pushed just under a blue water surface while three green arrows press up on its underside, showing the water shoving back.

And here's the trick: the more water a shape shoves aside, the harder the water pushes back. Tap to push this sealed, air-filled box under and watch.

tap to shove it under

Compare sealed boxes held at the same depth. A wider box encloses more space and pushes aside more water. Try changing its width.

how wide: narrow
narrowwide

More enclosed space underwater → more displaced water → bigger up-push.

Now take ONE lump of steel. Tap to reshape it: squeezed into a tight ball, or spread into a hollow boat wrapping a pocket of air. Same steel, same weight.

tap to reshape
shape: tight ball

Line them up over the water.

Side by side above blue water, a compact grey steel ball and a wide hollow steel boat wait with a clear gap beneath each. The boat is empty and dry; fine dashed gold air currents curl inside its open cavity.

The hollow boat wraps a pocket of air. Keep its shape fixed for a new test: what changes if water fills that pocket? 🤔

You just saw: a wider shape shoves aside more water, so water pushes back harder.

The hollow boat is floating. A leak lets water fill its air pocket. Without changing the steel or the outside shape, what happens?