Final answer:
A controlled experiment is when scientists test one variable at a time and hold all other variables the same. They manipulate the independent variable and measure the changes in the dependent variable. The control group is used as a baseline for comparison.
Step-by-step explanation:
When scientists test one variable at a time and hold all other variables the same, this is called a controlled experiment. In a controlled experiment, the researcher manipulates the independent variable and measures the changes in the dependent variable. The control group is a group that does not receive the experimental treatment and is used as a baseline for comparison.