You’ll need

  • A big plastic tub
  • Water
  • Things that float or sink, like rocks, sticks, Lego, ping-pong balls and golf balls
  • A tall, small plastic cup
  • A permanent marker
  • A plastic lid to a takeaway container
  • Materials to make your boat. Something that floats, like sticks, a plastic container or aluminium foil, and tape, rubber bands or string
  • Masking tape

What to do


Adult supervision is required. Water can be dangerous. Make sure that children are always supervised when near water.

Part A

  1. Place your materials on a flat surface.
  2. Fill your plastic tub about three-quarters full of water. (You can also do this part of the experiment during bathtime.)
  3. Think about what might happen when you drop the things you have collected into the water. Do you think they will float or sink? Put the different objects in the water to see which of them float or sink. See if your guess was right.
  4. Sort the objects into 2 groups: floating and sinking. Think about what makes these objects similar and different, and why some float and some don’t.
  5. Now make sure the plastic cup is about three-quarters full of water. Mark the water line with a marker.
  6. Drop rocks in one by one. Watch what happens to the water level.

Part B

  1. Using the same tub of water as for part A, place the plastic lid on top of the water so it’s flat on the water. What happens?
  2. Now try putting the plastic lid in the water again. But this time, put it in the water on its side. Now what happens?
  3. Make a boat out of something that floats. You may need to try a few different things, or try making several boats to see which is best!

Questions to ask

Part A

Could you guess which objects will float and which will sink? What did the things in the 2 groups have in common?

Where else can you see water levels changing? Try the experiment again, but with you in a bathtub! (Instead of marking the level with the permanent marker, use masking tape)

Part B

Why does the plastic lid float one way but not the other? Try the same thing with other things you think float.

How much weight can your boat carry? Try placing rocks on your boat. How many rocks can it hold?

Can you improve your boat design so it holds more weight? Experiment with:

  • shape – design, build and test different shaped boats. Try boats with sides or flat rafts. You could compare boats with flat and rounded bottoms. How does the shape of a boat affect how it floats or how much weight it can hold?
  • materials – design and build boats using a variety of materials, like a plastic container or sticks tied together to make a raft. Does the material you use float or sink? How does this affect how well the boat floats? Test each boat to discover which materials work best
  • size – design, build and test differently sized boats. How does the size of the boat affect how it floats or how much weight it can hold?

What's happening

When something goes into water it pushes water out of the way to make room for itself. This is called displacement. To float, an object must displace water that weighs as much as the object. Boats are designed to have a shape that displaces a lot of water. When a boat sits on water, it displaces its own weight in water. The displaced water pushes up, allowing even heavy boats to float.

Objects that are very dense (which means they are heavy for their size) cannot displace enough water to float. Light objects can. That’s why a ping-pong ball floats and a golf ball sinks. They are about the same size and shape, but one is much heavier than the other. That makes it denser, and it sinks.

Some ships are so huge it seems like they shouldn’t float! But, the ocean is waaaaaay bigger than any ship, so there is always enough displaced water to create the upwards pressure needed to float a boat.