Spontaneous Generation Hypothesis (disproven) stating that life could arise from nonliving matter.
Controlled Experiment A test of the effect of a single variable by changing it while keeping all other variables the same.
Manipulated Variable Factor in an experiment that a scientist purposely changes; also known as independent variable.
Responding Variable Factor in an experiment that a scientist wants to observe, which may change in response to the manipulated variable; also known as a dependent variable.
Theory Well-tested explanation that unifies a broad range of observations.
Key Concept How do scientists test hypotheses? Scientists test a hypothesis by an experiment in which only one variable is changed at a time. All other variables should be kept unchanged, or controlled.
Key Concept How does a scientists theory develop? As evidence from numerous investigations builds up, a particular hypothesis may become so well supported that scientists consider it a theory.