If each interior angle of a regular polygon is $144^{\circ }$, find the number of sides of the polygon:

The sum of an interior angle and an exterior angle of a regular polygon is always $180^{\circ }$.
$$\text{Exterior angle}=180^{\circ }-144^{\circ }=36^{\circ }$$

The sum of all exterior angles of any regular polygon is always $360^{\circ }$.

$$\text{Number of sides }(n)=\frac{360^{\circ }}{\text{Exterior angle}}=\frac{360^{\circ }}{36^{\circ }}=10$$