* Polar vs. Nonpolar: Salt (sodium chloride, NaCl) is an ionic compound, meaning it's composed of charged ions (sodium and chloride). It's very polar, meaning it has a strong attraction to other polar molecules like water.

* Mineral oil is nonpolar: Mineral oil is a mixture of hydrocarbons, which are nonpolar. Nonpolar molecules don't have a strong attraction to charged ions.

Like dissolves like: A general rule of thumb in chemistry is that "like dissolves like". This means polar substances dissolve in other polar substances, and nonpolar substances dissolve in other nonpolar substances. Since salt is polar and mineral oil is nonpolar, they won't mix.

If you were to try to mix salt and mineral oil, the salt would simply settle to the bottom of the container.