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Why do some things sink in water, and some things float?

When something pushes water out of the way, whether it sinks or floats depends on its weight.

When you drop something into water, it takes the water's place and pushes the water aside. If the thing weighs the same as the water it pushed away, or less, the water pushes back and it floats. But if the thing is heavier than the water it pushed away, it sinks.

A stone is small, but it is very packed and heavy, so it sinks. A big ship is very heavy, but it is hollow inside and pushes away a lot of water, so it floats. Shape matters too. If you roll some clay into a ball, it sinks. If you shape the same clay like a flat boat, it floats, because it pushes away more water.

Balloons and things like wood float because they are light and full of tiny holes. An iron nail is small, but it is very packed together, so it sinks. So it is not just about how heavy something is, it is about how much space that weight takes up.

Quick facts

When something goes into water, it pushes away water equal to its own size.
If something is denser than water, it sinks. If it is less dense, it floats.
Ships are heavy, but they float because they are hollow inside.
The same lump of clay can sink as a ball but float as a boat shape.
An iron nail is small, but it sinks because it is very densely packed.
Archimedes was the wise man who first explained how things behave in water.

Check what you remember

4 questions — all of them come from the quick facts above. There is no score and no wrong turn.

1. Why does something float?

2. Why don't ships sink?

3. What happens if we shape the same clay like a boat?

4. Who first explained why things sink and float in water?

Where this comes from

1 tr.wikipedia.org 85/100
Buoyancy - Wikipedia
An encyclopedia page that explains simply why water pushes on things.
2 tr.wikipedia.org 80/100
Density - Wikipedia
Explains how something's weight is measured against its size.
3 tr.wikipedia.org 75/100
Archimedes - Wikipedia
Tells the story of the wise man who first explained sinking and floating.

⚖️

What Is Density?

Density tells you how heavy something is for its size. In water, things that are denser than water sink, and things that are less dense float.

🚢

How Do Ships Float?

Ships are made of iron, but they are hollow inside. Because of this empty space, they push away a lot of water, and that makes them float.

🧠

Archimedes' Discovery

Long ago, a wise man named Archimedes found out that when something goes into water, it pushes away water equal to its own size. This rule is still named after him today.

🌊

The Mystery of the Dead Sea

The Dead Sea has a lot of salt in it. This makes the water denser, so people float on it easily and do not sink.

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