Fair tests — Science, 11–13 years
A fair test changes one important thing while keeping the others steady, so evidence can answer a clear question.
One change at a time
Suppose you want to know whether more light helps a plant grow. Change the light, but keep the plant type, water, soil and time the same. Then any clear difference is more likely to be linked to light rather than to several changes mixed together.
Why control the variables?
The problem is that the world changes for many reasons at once. If you give one plant more water and more light, a taller plant cannot tell you which change mattered. Keeping conditions steady makes a comparison fair and helps other people repeat the test.
Testing salt and ice
Question: does salt make ice melt faster? Put one ice cube on each of two plates. Add one teaspoon of salt to one cube and nothing to the other, then start a timer. After 20 minutes, measure the melted water: 18 ml with salt and 7 ml without it. The test supports the idea, though repeating it would make the evidence stronger.
A result is not proof
A common mistake is to test once and announce a rule. That feels reasonable because one clear difference looks convincing, but chance, inaccurate measuring or a hidden difference may have affected it. Scientists repeat tests, record all results and say what the evidence supports, not more than it supports.
Making comparisons in life
Fair comparisons help when choosing a torch, testing which soil suits a herb or checking whether a cleaning product works. They also matter in medicine and engineering, where changing several things at once can hide danger. Everyday decisions improve when you ask what was changed and what stayed the same.
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