Here's why:

* Polarity arises from unequal sharing of electrons: Polar molecules have an uneven distribution of electron density, creating partial positive and negative charges within the molecule.

* Covalent bonds are required for unequal sharing: Covalent bonds involve the sharing of electrons between atoms. The unequal sharing in polar molecules occurs when the atoms involved in the bond have different electronegativities, meaning one atom attracts the shared electrons more strongly than the other.

Important Note: Not all covalent bonds are polar. If two atoms with similar electronegativities form a bond, the electrons are shared fairly evenly, resulting in a nonpolar covalent bond.

Example:

* Water (H₂O) is a polar molecule. The oxygen atom has a higher electronegativity than the hydrogen atoms, leading to a partial negative charge on the oxygen and partial positive charges on the hydrogens.

* Carbon dioxide (CO₂) is a nonpolar molecule even though it has covalent bonds. The two oxygen atoms pull on the shared electrons equally, resulting in a balanced distribution of charge.