  In this paper we give a relation between the volume of sublevel sets and the
area of level sets using a Gelfand-Leray form. As a consequence, we give an
estimation of the volume of sublevel sets. In particular we give a proof of the
known fact that the derivative of the volume of a $n$-dimensional ball with
respect to the radius equals the area of the sphere which bounds the
$n$-dimensional ball.
