Here's why:

* Rock salt is primarily sodium chloride (NaCl). Sodium chloride is an ionic compound, meaning it's made of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-).

* Water is a polar molecule. This means it has a slightly positive end and a slightly negative end due to the uneven sharing of electrons.

* The attraction between the water molecules and the ions in rock salt is stronger than the attraction between the ions themselves. This causes the ions to break apart and become surrounded by water molecules, which is what we call dissolving.

The rate of dissolving depends on factors like temperature (warmer water dissolves salt faster) and how finely ground the salt is (finer particles dissolve quicker).