Based on observations, or data in the scientific literature, scientists formulate a hypothesis. It is possible to prove a hypothesis false, but it cannot be proven true. Scientists observed prevalence of dark-colored moths on sooty trees in polluted regions. If birds selected moths by color, then if equal numbers are released, birds will eat mostly one color. Conclusions from new data support hypothesis that birds are responsible for distribution.
Some investigations are controlled experiments.
Experimental variable is the step that is deliberately manipulated. Dependent variable is component of an experiment that changes due to the experimental variable.