A hypothesis is a testable proposition used to explain scientific phenomena. It may be based on previous observations (a sort of educated guess) or it may simply be a hunch. Most scientific hypotheses are combinations of observation and intuition. The important thing is that the hypothesis be testable. If the hypothesis is disproved through testing, then it is discarded and a new hypothesis is proposed. If the hypothesis stands up to rigorous testing, then our faith in the hypothesis is increased. This system of careful, rigorous, and unbiased testing of a hypothesis is what characterizes the scientific method.
