If you can pour solids - like flour or sugar - and liquids like water or oil, what's the difference between them?
Well, if you look closely at the flour or the sugar, you
will see that it is made up of tiny pieces - solid pieces - just as solid
as a lump of metal or a rock. In fact, a salt crystal and a big lump of
rock salt are both the same material - it's just that one is bigger than
the other. And you wouldn't argue that a chunk of rock salt was a liquid!
Solids can become liquids when they melt. They are still the same material, but they have changed state. They can harden and solidify again.
Some solids can dissolve in liquids - and some liquids can dissolve in other liquids. You know they have dissolved because the two are totally mixed. In any drop of the solution, you will find both materials. Often, the solution is clear - light passes through it. There are no lumps to stop it.
Some solids do not dissolve in liquids. They may sink to the bottom. They may float in the liquid. They are undissolved. You can filter them out.