Why do things float or sink?

Drop a stone in the bath and it sinks straight down. But a giant heavy ship floats happily on the sea. That seems backwards! How can a big ship stay up while a little stone goes down? Let us find out the secret of floating and sinking.

Water pushes back up

When you put something into water, it has to push the water out of the way to make room. And the water pushes right back, lifting up on the object. This upward push is called buoyancy.

If the upward push is strong enough to hold the object up, it floats. If the object is too heavy and the push is not enough, it sinks.

Light for its size floats

The trick is not just how heavy something is — it is how heavy it is for its size. A thing that is light for its size pushes aside lots of water and floats. A thing that is heavy for its size, like a stone, sinks.

A cork is light for its size, so it bobs on top. A coin is heavy for its size, so it drops to the bottom.

Shape matters too!

Here is the clever bit. A ball of clay sinks. But squash that same clay into a wide bowl shape, and it can float! By spreading out, it pushes aside more water and catches more of that upward push.

That is the secret of ships. They are big and hollow, so they spread out over lots of water and float, even when they are made of heavy metal.

Wonder fact: You float more easily in the sea than in a swimming pool, because salty water gives an extra-strong upward push!