Here's why:

* Water molecules are polar: They have a slightly positive end and a slightly negative end. This allows them to dissolve many ionic compounds (like salts) and form ions.

* Ions carry the charge: It's the presence of free ions (charged particles) that allows electricity to flow through a substance.

* Pure water has very few ions: In its purest form, water contains only a tiny number of hydrogen and hydroxide ions due to the auto-ionization of water molecules. This means there aren't enough charge carriers to conduct electricity effectively.

However, the presence of impurities can drastically change water's conductivity:

* Dissolved salts: When salts dissolve in water, they break down into ions, significantly increasing the water's conductivity. This is why seawater, which contains many dissolved salts, is a good conductor of electricity.

* Other impurities: Even small amounts of dissolved minerals, acids, or bases can increase water's conductivity.

The bottom line:

* Pure water is a poor conductor of electricity.

* Impurities in water, particularly dissolved salts, significantly increase its conductivity.