A controlled experiment is a scientific test where researchers change one variable while keeping all other conditions the same. This helps them understand how that specific change affects the outcome. For example, if a scientist wants to see how sunlight affects plant growth, they might grow two groups of plants—one in sunlight and one in the dark—while keeping factors like water and soil consistent. By controlling the environment, scientists can draw clearer conclusions about the effects of the variable they are testing. This method is essential in fields like biology and psychology, where understanding cause and effect is crucial for developing theories and solutions.