Factorize each of the following expressions:

$p^{2} q-p r^{2}-p q+r^{2}$

$=\left(p^{2} q-p q\right)+\left(r^{2}-p r^{2}\right) \quad[$ Grouping the expressions $]$

$=p q(p-1)+r^{2}(1-p)$

$=p q(p-1)-r^{2}(p-1)$                $[\because(1-p)=-(p-1)]$

$=\left(p q-r^{2}\right)(p-1)$      $[$ Taking $(p-1)$ as the common factor $]$