Flexi Says: In order to explain the observed phenomenon, scientists develop a number of possible explanations or hypotheses. A hypothesis is a suggested explanation for a phenomenon or a suggested explanation for a relationship between many phenomena. Hypotheses are always based on evidence that can be tested by observation or experimentation. Scientific investigations are required to test hypotheses. Scientists mostly base hypotheses on prior observations or on extensions of existing scientific explanations. A scientific hypothesis must meet two criteria: A scientific hypothesis must be testable, and a scientific hypothesis must be falsifiable.