A controlled experiment is a scientific test where one variable is changed while all other variables are kept constant. This allows researchers to determine the effect of the changed variable on the outcome. For example, if a scientist wants to test how sunlight affects plant growth, they would keep all other conditions, like water and soil type, the same while varying the amount of sunlight each plant receives. In a controlled experiment, there are typically two groups: the experimental group, which receives the treatment or change, and the control group, which does not. This comparison helps to isolate the effects of the variable being tested, ensuring that any differences in results can be attributed to that specific change.