Here's why:

* Controlled Experiment: A controlled experiment is designed to test the effect of a single variable (the manipulated variable) on a specific outcome.

* Manipulated Variable: This is the factor that the scientist intentionally changes.

* Identical Conditions: All other factors in the experiment are kept exactly the same (controlled) to ensure that any changes observed are due solely to the manipulated variable.

Example:

A scientist wants to test the effect of fertilizer on plant growth. They would:

1. Manipulated Variable: Set up two groups of plants, one with fertilizer and one without.

2. Controlled Variables: Make sure both groups receive the same amount of water, sunlight, and soil type.

3. Observe: Compare the growth of the plants in each group.

By controlling all other variables, the scientist can confidently attribute any differences in plant growth to the presence or absence of fertilizer.