A hypothesis must have a clear prediction along with a rationale for that prediction. This can written in the "If...then...because..." format. For instance, a good hypothesis could be "if the temperature rises above 100°C then the water will boil because water's boiling point is 100°C.
The whole point of the experiment then is to test the hypothesis. By the end of the experiment there will either be supportive evidence for the hypothesis or a lack thereof. Either way, it was a testable hypothesis.
