How do you know if an experiment is fair?

In one line

The rule

An experiment is fair if only one variable (the thing you change on purpose) is changed at a time, while all other conditions are kept the same.

Let's understand it simply

Imagine you want to know which soil is best for growing tomato plants. If you use different pots, give one plant more water, or put one in the sun and one in the shade, you won't know if the soil made the difference. You must keep everything the same except the soil. This way, you can be sure that any change in the plant is because of the soil, not something else.

How it works

1. Decide what you want to test (the variable).
2. Change only that one thing in your experiment.
3. Keep all other things (like water, sunlight, pot size) the same for all groups.
4. Observe and measure the results carefully.
5. Compare the results to see if the change made a difference.

The story behind it

The idea of a fair experiment became important when scientists wanted to be sure about their results. In the 1600s, people like Galileo and later scientists used careful testing to find out what was true. They learned that if you change too many things at once, you can't tell what caused the result. This is called the 'fair test' principle in science.

Where you see it in real life

• Cricket bat testing: Trying two bats, but using them on the same pitch, with the same bowler, and same type of ball. Only the bat changes.
• Cooking rice: Testing which brand cooks faster, but using the same amount of water, same stove, and same pot. Only the rice brand changes.
• Fertilizer for plants: Giving two plants different fertilizers but keeping sunlight, water, and pot size the same. Only the fertilizer changes.
• Pressure cooker whistle: Testing which cooker whistles first, but using the same amount of dal and water, and same gas flame. Only the cooker changes.

Why it matters

• Helps you find the real reason for a result.
• Makes your experiment trustworthy—others can repeat it and get the same answer.
• Needed for science exams and projects.
• Used in jobs like medicine, farming, and engineering to test what really works.

Common mistakes

• Changing more than one thing at a time (wrong: changing both soil and water for plants).
• Not controlling hidden factors (like one plant getting more sunlight).
• Forgetting to repeat the experiment to check if the result is the same each time.

Remember it

Fair means: Change only one thing, keep the rest the same.