Final answer:
A valid scientific experiment requires at least two variables: an independent variable to manipulate and test its effects, and a dependent variable to measure the outcomes of those manipulations.
Step-by-step explanation:
The minimum number of variables necessary to conduct a valid scientific experiment is two: a manipulable (independent) variable and a responding (dependent) variable. The independent variable is the one that is changed or controlled in the experiment to test the effects on the dependent variable. Meanwhile, the dependent variable is the variable being tested and measured. It is crucial to have at least one of each to establish a cause-and-effect relationship. Additionally, control variables should be identified and kept constant to ensure that the results are due to changes in the independent variable alone and not other factors.
An experiment generally tests how the dependent variable is affected by the independent variable. For instance, if a scientist is testing the effect of light on plant growth, the intensity of light would be the independent variable, while the growth of the plant is the dependent variable. Control variables in this example could include soil quality, water, and pot size, all of which need to be held constant to ensure a valid experiment.