Early Years Science: Floating and Sinking

Children can investigate the behaviour of objects placed in water with these activities from Linda Thornton and Pat Brunton


Water has many fascinating properties that young children love to explore, in the water tray, paddling pool, in a puddle, in the bath or even in the loo! As part of this experience, investigating whether things float or sink happens frequently, either by chance or by design.

Understanding why some things float and others sink is not as simple as it appears. Having lots of opportunities to experience 'floating, sinking and sunk' is the best way to build up this aspect of scientific knowledge.


What do I need to know?

Use the background information below to inform your planning and help you support young children's scientific learning. It is not intended that children in the Foundation Stage are taught these facts.


When a boat is placed in water it takes up space, just as it does in the air. To make this space the boat displaces a volume of water. The amount or volume of water displaced by the boat is equal to the volume of the boat below the water line. The lower down in the water the boat sits, the more water it displaces.

You can see the effects of displacement easily if you fill a container almost to the brim with water and then gradually add small pebbles to it.

The water level will rise and eventually spill over the top because the pebbles have displaced some of the water.


When the water tray contains water, there are forces working in all directions. There is a downward pressure through the effect of gravity, sideways pressure on the sides of the water tray and upwards pressure pushing towards the surface. This upward pressure is called upthrust.

If you gently push an inflated balloon down on the surface of the water you can feel the upthrust pushing back. If you empty a plastic bottle, screw the lid back on tightly and then hold it under water, the force you need to apply to hold the bottle down is equal to the upthrust of the water pushing it upwards.


In the first article in this series, we talked about objects and people staying still when the forces acting on them are the same in all directions. We call these balanced forces.

When a boat is floating in water, the upthrust acting on the boat is equal to the weight of the volume of water displaced. The boat floats because the forces acting on it are balanced. The force of gravity is acting downwards and the force of upthrust is acting upwards.

Making floating things sink

If you add passengers or cargo to a boat, its weight will increase. The boat will float lower in the water because it is heavier. The volume of water displaced by the boat increases, so the force of the upthrust increases to balance it. If you add even more weight to the boat there will come a point where the total weight of the boat is greater than the upthrust from the water and the boat will sink to the bottom.

Making sinking things float

Changing the shape of an object will affect whether it will float or sink.

For example, a ball of Plasticene placed in a bowl of water will sink. If you now make the same ball of Plasticene into a boat shape and place it gently on the water, it will float. This is because the volume is now greater, so it displaces a greater volume of water. You can see the same thing happening in the swimming pool - if you curl up into a ball you sink and if you spread your arms and legs out, you float. This is also the reason why a lump of iron will sink to the bottom, but the same iron shaped into the hull of a boat will float on the surface.


Floating and sinking

What you need

Clear container of water - water tray, bucket or bowl. You could use a paddling pool outdoors. Selection of objects that float or sink, such as corks, lolly sticks, lids, foil dishes, cotton reels, plastic and metal teaspoons, marble, conker, coin, bottle tops, sponge. Include different fruits and vegetables to see whether they all behave in the same way.

What to do

You may offer many of the suggested materials as part of your continuous provision, but organising an adult-led activity which investigates floating and sinking is an ideal way to introduce children to predicting and testing their ideas.

  • Working with a small group of children, ask questions such as: 'What does the word float mean?' 'What do we mean when we say something sinks?
  • Find out more about the children's experiences and ideas about floating and sinking by asking: 'What objects can you think of that sink to the bottom of the water tray?' 'What objects stay afloat?'
  • Together, look at the range of different objects.
  • Encourage the children to handle the objects in turn and to predict whether they think the objects will float or sink. Ask questions such as: 'What is it made of?' 'Will it float or will it sink?' Can you think why?' 'Do you think bigger things always sink?'
  • Help the children to test each of the objects in turn to see if they float or sink. Ask questions such as: 'Was your guess (prediction) right?' 'Do some objects always float?'
  • You can record the results of the children's investigation either by drawing the objects on two charts labelled 'objects that sink' and 'objects that float', or simply by placing the objects in separate trays.


Who sank the boat?

What you need

Who Sank the Boat? by Pamela Allen (Puffin Books), water tray or paddling pool, small food containers for boats, small-world play animals and people.

What to do

  • Read Who Sank the Boat?.
  • Look carefully at the pictures to see what is happening. Draw attention to where the animals sit in the boat to try to balance their weight. Ask: 'What happened to the boat when the cow got in? Why?' 'Can you find pictures where the boat is balanced?'
  • Read the story again and focus on observing what is happening to the boat in the water. 'Who did sink the boat?' 'Why do you think that happened?'
  • Encourage the children to act out the story in the water using their small-world toys and their 'boats'. 'How many toys can you fit in the boat before it starts to sink?' 'What happens if the passengers are all on one side of the boat?'

Extension ideas

  • The children could invent 'floating, sinking and sunk' stories of their own using other characters, such as pirates or dinosaurs. You might want to provide the tools and materials for the children to make their own boats.
  • Think about providing individual containers of water so that each child has the opportunity to concentrate on manipulating and positioning the toys and boats with care.
  • Read the stories Mr Archimedes' Bath by Pamela Allen and Mr Gumpy's Outing by John Burningham as starting points for exploring displacement and floating.


Vocabulary to introduce

float sinking sunk

big small shape

heavy light weight

water bottom surface

boat side balance


Further information

  • Science for Primary Teachers - An audit and self-study guide by G A Peacock (Letts Educational)
  • Who Sank the Boat? by Pamela Allen (Puffin Books)
  • Mr Archimedes' Bath by Pamela Allen (Puffin Books)
  • Mr Gumpy's Outing by John Burningham (Red Fox)

Linda Thornton and Pat Brunton are education consultants with a special interest in science and technology for young children. Visit their website at www.alcassociates.co.uk

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