Sam wants to know if plant food helps bean plants grow. He gives one plant food and leaves one without. But he also puts the fed plant in the sunny window and the other in a dark corner. When the fed plant grows taller, what made the difference?
He cannot tell! He changed two things at once: the food and the light.
Key idea
In a fair test you change only one thing (the thing you are testing), keep everything else the same, and measure the result. Do it more than once to be sure.
| Change | Keep the same | Measure |
|---|---|---|
| Plant food or none | Light, water, soil, pot, kind of seed | Height in centimeters |
Worked example: Find the problem
Lily tests which ball bounces higher. She drops the red ball from her waist and the blue ball from over her head.
- She is testing the ball, so only the ball should change.
- But the drop height changed too, and a higher drop makes a higher bounce.
Answer: Not fair. Drop both balls from the same height onto the same floor.
Common mistake
Changing two things at once, or testing only one time. One plant can be sick by bad luck; three plants each way gives a fairer answer.
Try it at home
- Get two paper towels of different brands and two cups of water with the same amount.
- Dip the corner of each towel into the water for the same number of seconds.
- Measure how high the water climbs on each with a ruler. Which brand soaks up more? What did you keep the same?
Going further. In the Pendulums lab, change only the length, then only the weight, and see which one changes the swing.