A scientist uses knowledge and a logical approach to solve a problem or explain an observation. Scientists use instruments to study forces, disease, and materials. Without scientists, we would not have gone to the Moon, discovered penicillin, or developed computers.

Scientists create a theory to explain observations. Theories are often written as equations. A good theory makes predictions that can be tested by further observations and experiments. A trial theory is called a hypothesis. If its predictions are correct, the hypothesis is supported.

An experiment is a practical test of a theory. By performing experiments, scientists seek answers to questions. They use measuring instruments to record the results of their experiments as numbers.