This interesting activity has been adapted from [url=https://tube.geogebra.org/orchiming]Antony C. M. Or[/url]'s applets. The function [math]z=x\cdot y\cdot (x^2-1)\cdot(y^2-1)[/math] is presented in the middle. The contour lines can be observed on the left while the plane [math]z=c[/math] intersects the function. [i]c[/i] can be dynamically changed by dragging it with the mouse on the right.