The first step of the scientific method is observation of a problem (also known as posing a problem). This problem is something that has not been previously answered by science before or is an exception to a proven idea. For example, in the late 18th century, Antoine Lavoisier observed that tin grew more massive as calx formed on it. Thus, Lavoisier observed a problem because, according to the prevailing phlogiston theory (a theory which held that an element called phlogiston was released when elements were burned) this meant the tin was actually releasing a negative amount of phlogiston for it to be gaining weight.
After posing a problem, the next step in the scientific method is to form a hypothesis, which is a prediction of the solution or cause of the problem. Lavoisier's hypothesis was that a factor in the air, rather than a factor in the flammable material, caused materials to gain or lose weight when they were set on fire. Hypothesis used to be the final step in the scientific method for thousands of years. Ancient Greek philosophers and scientists would often hypothesize without verifying their statements.
Once a hypothesis has been formed, it must be experimented in order to determine its accuracy. Lavoisier tested his hypothesis by burning mercury in various levels of air pressure. He found that different amounts of calx formed depending on the amount of air, thus proving that it was the air rather than an inherent part of the substance that changed a burning substance. Experiments must be repeated several times in order to factor for human error.
Once an experiment has yielded a result, the hypothesis can either be verified or falsified. If it is verified, a theory can be formed. A theory is a scientific statement that has been verified through multiple experiments. Lavoisier's theory was verified, and he published his Theory of Combustion (also known as the Oxygen Theory) in 1789. Theories should be presented so that other scientists can re-test them. If an inconsistency is found, the theory is invalid and the steps for the scientific method begin anew.