Sink or Float?

Our Standard 3 students are learning to be scientists through predicting, investigating and drawing conclusions.


Scientists try to understand our world and how things work. One thing scientists do is make predictions and then test to see if their predictions were correct or not. A prediction is a guess based on what we already know or understand. Standard 3 students were given some different items such as a tennis ball, piece of blu-tack, a stick and a rock. They had to predict whether the item would sink or float.


At first, the students thought that if an item was big it would sink and if it was small it would float. This was sometimes true. George predicted the piece of wood would sink because it was big but it didn’t sink, it floated. The blu-tack, that was smaller in size than the piece of wood sank straight away to the bottom. After completing the experiment the students then had to rethink. Why is it that some items sink and others float? If it isn’t about the size then what is it??


The students then read a few different books and learned about density. If an item is very dense then it will sink.

The students then investigated another question: Can you make a rock float?

Just as scientists do, the students made models, to answer the question. The students worked on rafts and boats that the rock could sit on so that it wouldn’t sink. They used toilet rolls, straws, cardboard and tissue paper to make their models. We had a couple of boats that sank to the bottom with the rock but most of them floated and so the rock was safe.







Wanafunzi Questions

 1. What are some of the things that scientists do?

2. What is a prediction? What is an example of something a scientist could make predictions about?

3. What does it mean if something floats?

Wanafunzi Lesson Ideas

Below are some lesson ideas for teachers to use. We also want to help you link this blog to the curriculum in your country. Our aim is for our Wanafunzi program to be integrated into your curriculum and hopefully make it easier for teachers to use in the classroom with students of all ages and abilities. Please email [email protected] if you have any suggestions or visit if you have further queries about our Wanafunzi program.

1. Conduct a similar lesson with your students. Select 10 objects and get the students to predict if the object will sink or float. Ask the students to explain their thinking. After the experiment view the video above and explain what density is to the students. After the experiment have students predict whether some other objects will sink or float based on what they have learned.

2. Get students to make a boat that can carry the heaviest rocks or can travel the fastest. Have students predict why their boat was successful or not successful.

3. After completing the experiment students can create a poster or write a report to explain why some things sink and others float.

Curriculum Links

Australian Curriculum - Grade 2 - Science

  • Science involves asking questions about, and describing changes in, objects and events
  • Participate in different types of guided investigations to explore and answer questions, such as manipulating materials, testing ideas and accessing information sources
  • Use a range of methods to sort information, including drawings and providing tables

Australian Curriculum - Grade 3 - Science

  • Science involves making predictions and describing patterns and relationships
  • Suggest ways to plan and conduct investigations and find answers to questions
  • Use a range of methods including tables and simple column graphs to represent data and to identify patterns and trends
  • Compare results with predictions, suggesting possible reasons for findings
  • Reflect on the investigation, including whether a test was fair or not.
  • Represent and communicate ideas and findings in a variety of ways such as diagrams, physical representations and simple reports.


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