A couple of things need to be discussed.
First of all, a hypothesis is a prediction that you make based on what you already know.
For example: "If I put a string into a jar full of salt water, it will eventually form crystals."
I think you are asking for the explanation, though. Nearly anything that dissolves in water forms what are called ions. Ions are positive or negative particles. Only a certain amount of ions can stay dissolved in water. If the water gets "overloaded, the ions start to stick together. Usually, they start by sticking to some object that is near them. String has a lot of surface for the ions to stick to, so that is the surface they choose. Once a few ions start to stick, others follow.
Before you know it, so many ions have come out of the water that a crystal has formed!