Change one thing
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What you will learn
By the end, you can plan a fair test, choose what to measure, read a results table and say what the evidence shows.
Mila thinks more water makes cress grow taller. Pot A gets lots of water on the sunny windowsill; pot B gets a little, inside a cupboard. Pot A wins easily. Her brother Tomas says: "That proves nothing." Annoyingly, he is right.
Read the explanation
A scientist starts with a question, then tests it. In a fair test you change only one thing (the variable), keep everything else the same, and measure the result. If two things change at once, you cannot tell which caused the difference. The answer comes from the evidence, not from what you hoped.
- Use units: centimetres, millilitres, seconds, degrees Celsius.
- Repeat measurements; one odd result is not a rule.
- Write down every result, even the surprising ones.