All molecules are held together by electric forces. Normally water molecules don’t conduct electricity very well because they are mostly neutral. When salt crystals dissolve in water, however, salt molecules come apart as they mix with water molecules, and (in the case of salt) each molecular piece carries an electric charge rather than being neutral.
It follows that saltwater conducts electricity well because there are abundant charge carriers—namely the salt ions such as Na+ and Cl-. (This differs greatly from metals, where a sea of free electrons are the charge carriers.) This is important because animal cells, like neurons and muscle cells, generate and conduct electricity using salt ions, i.e., “saltwater.” Usually biological voltages are minuscule but don’t grab an electric eel!
Later she can learn about ionic bonds & compounds, and electro-chemistry etc.