Nobody knows how ideas appear, but worrying over a problem and having it present in the mind for many hours leads to the sudden appearance of a solution (or an explanatory hypothesis).
The questions which arise from an observation of: Why? How? What factors or variables explain the phenomenon? etc. are answered with a hypothesis.

A hypothesis is an answer in advance, which is presented as a possible explanation to a problem which arises when trying to explain a phenomenon and which should be verified by experimentation. 

The facts will decide (will demonstrate it)!

Without a first hypothesis there can be no working plan. First hypotheses are of two types:

• Hypotheses of how to set up useful experiments and how to design appropriate apparatus to carry out experiments or measure new magnitudes of the phenomenon under study. 
• Hypotheses of why and how some variables influence the phenomenon and others don't. 
  For example: The time that a pendulum takes to complete an oscillation MAY BE INFLUENCED  by the mass, the length of the pendulum, the distance from which we let it go, the colour of the material, the height from the ground, etc. 
  

All the hypotheses are designed following the reasoning that ALL CAUSES CREATE AN EFFECT.

Galileo assumed that although the mass of the pendulum varied it would take the same time to complete an oscillation because he had already proved that when he threw different masses from the Tower of Pisa they took the same time to fall to the ground.