Sink or Float?
Discover why some objects sink and others float by testing everyday household items in water.
Adult supervision recommended for younger children.
This experiment uses water and household items. A grown-up should help with setup and be present during the activity.
You Will Need
Step-by-Step Instructions
Set Up
Fill your large bowl or basin with water and place it on a towel. Lay out all your test objects on the table next to it.
Draw a Prediction Table
Draw a table with three columns: Object, My Prediction (sink or float?), and What Happened. Write down all your objects.
Predict First!
Before testing anything, look at each object carefully. Feel how heavy it is. Think about what it is made of. Write down your prediction: will it sink or float?
Test Each Object
Gently place each object on the surface of the water. Watch carefully. Does it float on the surface, or does it sink to the bottom? Record what happens in your table.
Compare Your Results
Look at your predictions and your results. How many did you get right? Were there any surprises? The apple is a tricky one!
Extension Challenge: Make Plasticine Float!
Take your ball of plasticine. It sinks, right? Now try to reshape it so it floats! Hint: think about the shape of a boat. Can you make a plasticine boat that floats on the water?
Sample Results Table
| Object | Prediction | Result |
|---|---|---|
| Coin | Sink | Sinks |
| Cork | Float | Floats |
| Apple | Sink? | Floats! |
| Key | Sink | Sinks |
| Sponge | Float | Floats |
What Happened?
Some objects sank straight to the bottom, while others bobbed happily on the surface. You probably noticed that heavy metal things like coins and keys sank, while lighter things like the cork and sponge floated.
But the apple was a surprise! Even though it is bigger and heavier than a coin, the apple floats. That is because floating is not just about how heavy something is. It is about how heavy something is for its size.
If you reshaped your plasticine into a boat shape, it probably floated. The ball of plasticine is heavy for its small size, so it sinks. But when you flatten it into a wide boat, it pushes away (displaces) more water, and the water pushes it up enough to float!
The Science Behind It
Density is the key idea here. Density means how much "stuff" (matter) is packed into a certain amount of space.
If an object is denser than water, it sinks. If it is less dense than water, it floats.
A coin is very dense: lots of metal packed into a tiny space. An apple has tiny air pockets inside it, which makes it less dense than water overall, so it floats!
When you shape plasticine into a boat, you spread the same amount of plasticine over a bigger area and trap air inside. This makes the overall density lower than water, so it floats.
Think About It
Why do you think a big heavy ship made of steel can float on the ocean, but a tiny steel nail sinks?
If you put a sealed empty bottle in the water, it floats. What would happen if you filled the bottle with sand? Why?
Some fruits float and some sink. Can you test different fruits and sort them? What do you notice about the ones that float?
How might knowing about floating and sinking help people who build boats or design life jackets?
Knowledge Check
Test what you have learned! Select the correct answer for each question.
Question 1
Which of these objects would most likely sink in water?
Question 2
An object floats in water because it is...
Question 3
Why does an apple float even though it is heavier than a coin?
Question 4
How can you make a ball of plasticine float?
Question 5
What does "density" mean in simple words?
Key Concepts Summary
- ● Objects float if they are less dense than water. They sink if they are more dense.
- ● Density means how much "stuff" is packed into a certain space — not just how heavy something is.
- ● A big, heavy object (like an apple) can float if it is not very dense for its size.
- ● Shape matters! Changing the shape of an object can change whether it sinks or floats.
- ● Good scientists predict first, then test, then record their results.