How Boats Float: Lesson for Kids

Instructor: Suzanne Rose

Suzanne has taught all levels PK-graduate school and has a PhD in Instructional Systems Design. She currently teachers literacy courses to preservice and inservice teachers.

Why do some things float and others sink? In this lesson, you'll find out why this happens and how it's related to what happens to the water when you get into a bathtub!

Sinking and Floating

Have you ever tossed a small pebble into a lake or river? What happens? It sinks, right? Have you ever wondered why that little pebble sinks to the bottom, but a huge boat made of metal floats on top of the water? Let's investigate and find out!

Every object that you put into water will either float or sink. It doesn't have anything to do with how big the object is. For example, you could put a metal spoon in water and it will sink, but a piece of paper as big as your house might float. So, size doesn't have anything to do with sinking and floating. What does?

Density and Water

Density is actually what decides whether an object sinks or floats. Density has to do with how much a certain amount of the object weighs. So, if you filled a shoebox with feathers and weighed it, and then filled the same size shoebox with pebbles and weighed it, they wouldn't have the same weight. The pebbles would weigh more. This means they are more dense than the feathers and have a higher density.

If we filled the shoebox with water and weighed it, and then compared that to the weights of the box of feathers and the box of rocks, we could predict which one would sink and which one would float. Because the box of rocks weighs more than the same amount of water, it would sink. Because the box of feathers weighs less than the same amount of water, it would float.

Archimedes and Displacement

Have you ever noticed that when you get into the bathtub, the level of the water gets higher? This is called displacement. The amount of water that is displaced by your body weighs the same amount as your body does! If you put a toy that weighs one pound into the bathtub, it will displace an amount of water that weighs one pound.


A scientist named Archimedes discovered displacement over 2,000 years ago as he was experimenting with water in his bathtub. Because of this, the idea of displacement is named the Archimedes Principle. As the object pushes the water away, the water pushes back on the object. This pushing force, called a buoyant force, is equal to the weight of the object. It's this buoyant force that makes an object float.

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