Why does the investigator in an experiment change only one varvariable

When people perform experiments of any sort, the professional people only change one variable so that they know what is being changed. For instance if I compared the effect of the growth of flies, and one group had more food and cleaner water, and the other group had neither, it would be pointless. It would be pointless because even if the experimental results were a success, you wouldn't know for sure which variable caused the change, the water or the food variable. That's why there can only be one man-made variable at a time.
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