To find the answers to scientific questions, scientists use an organized set of procedures called the scientific method. Often described in textbooks as a step-by-step process, the scientific method is, in reality, less prescribed than that.

A scientist begins by asking a question that can be answered by gathering evidence. Questions arise in all sorts of ways: for example, from observations, unexpected results, or the research findings of others. The scientist then develops an initial hypothesis, or possible answer to the question, arrived at based on past research or experience. In exploring the complex biology that enables cancerous tumors to grow, Dr. Judah Folkman formed his initial hypothesis based on decades of research. He suggested that tumors themselves can induce the formation of blood vessels, which in turn nourish the tumors. This phenomenon is called angiogenesis.

When planning an experiment to test a hypothesis, it is important to first identify all variables—the factors in an experiment that might affect the outcome. In a controlled experiment, all of the variables are kept the same except for one. The experiment would include a sample—the control—which is observed under normal conditions, and another sample that is subjected to the variable being studied for effect. For example, in his experiments, Dr. Folkman implanted a protein that he hypothesized would induce blood vessels to grow in corneal tissue. Because corneal tissue does not normally induce blood vessel growth, a sample of untreated corneal tissue could be used as the control against which the results of the protein implantation in a second sample of tissue could be measured.

Experiments should be designed so that results can be measured. Typically, scientists conduct the same experiment more than once to be sure the results can be replicated. What ultimately turned Dr. Folkman's early critics into competitors was their ability to repeat his experiments with the same results.

Finally, the results of experimentation should be analyzed and a conclusion drawn. Do the results validate the hypothesis? If not, do they contain ideas for further experimentation based on a modified hypothesis? After Dr. Folkman proved that angiogenesis occurred, he still needed to determine the mechanism behind it, so he designed a new experiment to confirm his thinking on that subject.

Determine the toxicity of salt to an intracellular liver process in this NOVA classroom activity.

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