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Depending on what you want to investigate, you will need a different test. For example, imagine you want to change the speed of your car.

If you want to see how the material of the tires affects the cars speed, this would need a comparative test. Things like materials are qualitative data.

If you want to change the weight of the car to see how this affects its speed, this would need fair test. You would change the amount of weight and record the changing speed. Anything that is measured in number, like weight or height, is quantitative data.

This test compared the ice cream in two environments. They observed that one ice cream melted more quickly than the other so it was a comparative test. This is a little bit different to a fair test.

When scientists do tests or experiments, they always only change one thing and record another. All other things are kept the same. This is a fair test.

If more than one thing changes in a test or experiment, this is not a fair test. We cannot know for certain what caused the changes.

You could carry out a fair test to see how heat affects the time it takes for ice cream to melt. Keep everything about the ice cream exactly the same (amount, type of ice cream, surface area) and only change the temperature. Record the time each takes to melt and this would be a fair test.

Watch this clip from where they carry out this experiment.

An investigation always starts with a question.

Write a fair test to answer this question: Do all plants grow better in soil?

Think about:

• What do you need to keep the same?
• What will you measure or observe?

Ask a responsible adult to help you with this activity.
You will need a ruler and several different types of ball, such as a tennis ball, a football and a golf ball.

1. Find a place where you can stand and drop a ball onto the same surface.
2. Ask the adult to use the ruler to measure how high the balls bounce.

What other scientific investigations could you complete using the same balls?