Question

One student did an experiment to find out if fertilizer can make plants grow taller. The steps of the experiment are listed below.

Take three pots of the same size.
Fill the pots completely with the same type of soil.
Plant tomato seeds in the pots of soil.
Add different amounts of fertilizer to the three pots.
Keep the first pot inside the house, the second pot under the shade of a tree, and the third pot in direct sunlight.
Add the same amount of water to the pots every day.
Measure the heights of the plants after regular intervals.
The results of the experiment were not valid. Wh

Answer 1

"Keep the first pot inside the house, the second pot under the shade of a tree, and the third pot in direct sunlight."
Helps to invalidate the hapless student's well meaning efforts.
Several other variables have been introduced. So, for example, if the sunlit pot grew tallest was it the fertiliser or was it the sunlight ?

Also, if the test is for fertiliser and the question is "does fertiliser alone help ?", then you want at least one REFERENCE pot with NO fertiliser, and then other pots with fertiliser. Should the fertiliser pots all grow taller than the ref pot ... optimistic, shall we say. And keep the pots together to avoid inadvertently introducing variables such as sunlight etc