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Fair Test Is . . .
To
make sure an investigation is a fair test, you need to keep
everything constant except the variable you're testing. Take
the question, Will tulips sprout earlier if we grow them
in compost or in regular soil? Students might plant one
tulip in each of six pots. Three pots will contain compost and
three will contain regular soil. Students must try to keep all
other variables - sunlight, moisture, type of bulb, planting
depth, and so on - constant. If they didn't - and planted some
bulbs deeper than others, for instance - they couldn't be sure
whether it was the soil type or bulb depth that made the difference.
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