Filtration works because the filter paper has tiny holes or pores in it. These are large enough to let small molecules and dissolved ions through, but not the much larger particles of undissolved solid.
Separating insoluble solids
Crystallisation is used to produce solid crystals from a solution. When the solution is warmed, some of solvent evaporates leaving crystals behind. For example, crystallisation is used to obtain copper sulfate crystals from copper sulfate solution.
To obtain large, regularly shaped crystals from crystallisation:
After the remaining solution has cooled down, pour the excess liquid away (or filter it). Dry the crystals using a warm oven or by patting them with filter paper.