Your child drops a toy in the bath and watches it bob back up, then asks why the soap sinks. It’s a great science question, and you can explore it with a bowl of water and a few kitchen things. Here’s a simple sink or float experiment for kids, why things really float (it isn’t just about weight), the plasticine boat trick, how to keep it safe, and what children learn about materials at school.
What do you need for a sink or float experiment?
Nothing special. A big bowl or the kitchen sink filled with water, and a handful of things to test. Questacon, Australia’s National Science and Technology Centre, suggests “balls, blocks, cutlery, leaves” and says to avoid “glass, electronics and valuables”. A good starter set:
- an apple
- a metal spoon
- a small piece of wood
- a coin
- a lump of plasticine or playdough
- a ping pong ball and a golf ball, if you have them
- an empty plastic bottle with its lid on
- a towel for spills
How do you do a sink or float experiment?
- Fill the bowl. Fill it most of the way with water and put it on a towel.
- Feel each thing first. Is it heavy or light? Hard or soft? Questacon’s first step is to feel each item before it goes in.
- Guess first. Ask your child to sort everything into a “float” pile and a “sink” pile, and say why.
- Test one at a time. Gently place each thing in the water and watch. Questacon adds a lovely question: what happens if you put it in the water a different way?
- Ball, then boat. Roll the plasticine into a tight ball and test it. Then squash the same plasticine into a flat boat with sides and test it again.
- Record and compare. Tick what really happened next to each guess. Which guesses were right? Which were surprises?
Wrong guesses are the best part. They’re exactly where your child starts to wonder why.
Why do some things float and others sink?
Two forces are at work. The Science Learning Hub, a science education site from New Zealand’s University of Waikato, explains that gravity “pulls the object down”, while the water being pushed aside “pushes upwards on the object”. If the upward push can match the object’s weight, it floats.
Whether that happens depends on the object’s density. Questacon puts it simply: density is “how heavy it is for its size”. Its example is perfect for kids: a golf ball is the same size as a ping pong ball but much denser, so the golf ball sinks and the ping pong ball floats.
That’s why size and weight alone don’t decide it. A huge log floats while a tiny coin sinks. The Science Learning Hub lists the common mix-ups children have, including “light objects float and heavy objects sink” and “small objects float and large objects sink”. Both sound sensible, and both are wrong, which makes them great to test.
| Test this pair | What happens | What it shows |
|---|---|---|
| Ping pong ball and golf ball | The ping pong ball floats, the golf ball sinks | Same size, different density |
| Empty bottle (lid on) and the same bottle full of water | The empty bottle floats, the full one sinks | Air makes the bottle lighter for its size |
| Ball of plasticine and a plasticine boat | The ball sinks, the boat floats | Same material, different shape |
Why does a plasticine boat float when a ball sinks?
This is the bit that amazes children. It’s the same plasticine, so what changed? The shape. The Science Learning Hub explains that “a ball of modelling clay will sink, but if you shape the ball into a boat shape, the clay will float”, because the boat shape takes up more room and pushes aside more water. Another way to see it: the boat holds air, so the boat and the air inside it together are less dense than water.
To picture the water being pushed aside, think of the water that spills over the edge of a completely full bath when you climb in. Big steel ships float the same way. The Science Learning Hub lists ships, dinghies and even soup bowls as things that float because “their shapes add air”, while anchors and stones sink.
Want a challenge? Questacon suggests loading a little boat with small toys, one at a time. As each one goes in, the boat sits lower, and when the push down is bigger than the water pushing up, it sinks. Try different boat shapes to see which carries the most. Our post on kitchen science experiments has a foil boat version that carries coins.
Is a sink or float experiment safe for little kids?
Yes, with a grown-up right there the whole time. Even a bowl of water needs care around young children. The Royal Children’s Hospital Melbourne says to “never leave a child alone around water” and warns that “it only takes 20 seconds and a few centimetres of water for a child to drown.” Its advice is to “empty baths, buckets and paddling pools immediately after use”, so tip the bowl out when you finish.
Coins are also easy for babies and toddlers to swallow. The same hospital lists coins among the things children may swallow and says to store small objects “out of reach of young children”. If there’s a baby in the house, count the coins in and out, or swap them for a metal key or spoon.
What do children learn about floating at school?
Floating and sinking is a classic way to explore materials in the early years. In Victoria, children learn across Foundation (Prep) to Year 2 that “objects can be made of one or more different materials; these materials have observable properties” (VC2S2U04). They also practise thinking like scientists: making predictions (VC2S2I01), following steps safely (VC2S2I02), observing and recording (VC2S2I03), and comparing what they saw with what they predicted (VC2S2I05).
In the national Australian Curriculum (AC9), the matching Foundation description asks children to “recognise that objects can be composed of different materials and describe the observable properties of those materials” (AC9SFU03). The idea of water pushing up comes later: a Year 4 example is “observing how the pushing force of a liquid enables an object to float” (under AC9S4U03). That push is a force, just like the pushes and pulls in our post on push and pull forces for kids. For the bigger picture of Year 2 science, see what a Year 2 child should know.
KidStudy’s Year 2 Science lesson Float or Sink? looks at what a thing is made of and its shape: wood floats and rocks sink, an apple floats, things full of air float, and the same plasticine sinks as a ball but floats as a boat. Its home activity, the Float or sink test, uses a bowl of water, an apple, a metal spoon, a piece of wood, a coin and a lump of plasticine.
Questions parents ask
Why do some things float and some things sink?
Gravity pulls an object down and the water pushes it up. If the object is less dense than water (lighter for its size), the water’s push can hold it up and it floats. If it is denser, it sinks.
Do heavy things always sink?
No. A huge log floats and a big steel ship floats, while a tiny coin sinks. What a thing is made of and its shape matter more than how heavy it is.
Why does a boat shape float?
A boat shape takes up more room in the water, so it pushes aside more water, and that water pushes back up. The boat also holds air, which makes the boat and its air lighter for their size than water.
Can a rock float?
Some can. The Australian Museum notes that “some rocks like pumice can float on water.” Most rocks sink. Our post on rocks and soil for kids has more on different kinds of rock.
Does an apple float or sink?
An apple floats. The Science Learning Hub lists apples among things that float because they are less dense than water. It surprises many children, so it’s a great one to guess first.
What age is a sink or float experiment for?
It works well in the early years of school. KidStudy’s lesson is written for Year 2, but younger children can sort and test with you, and older ones can explain their results and design a boat that carries more.
Sources
- Activity sheet: Floating and Sinking (Questacon at Home, K–2), Questacon
- Building Science Concepts: Floating and sinking, Science Learning Hub, University of Waikato
- Learn how to classify rocks, Australian Museum
- Water safety, The Royal Children’s Hospital Melbourne
- Swallowed objects, The Royal Children’s Hospital Melbourne
- Victorian Curriculum F–10 Version 2.0: Science, VCAA
- Australian Curriculum v9.0: Science, Foundation, ACARA
- Australian Curriculum v9.0: Science, Year 4, ACARA
