This demo shows a density function f(x,y,z) defined over a region in three-space, and it computes the total mass contained in the region. The region is bounded by the six surfaces x=a, x=b, y=c(x), y=d(x) q(x,y) and p(x,y), so you can change the region by entering different values for a and b, by entering different curves c(x), d(x) or by entering different surfaces p(x,y) and q(x,y).