Below are some examples of graphs of two-variable functions. (It may take a few seconds for the graphics to load.) These graphics are interactive, with zooming and rotation. If you place your mouse cursor in the frame, you’ll also see some curves appear on the surface. These are contour curves (sometimes called level curves). If the mouse cursor is at point $(a,b,c)$ on the surface, then these are the curves obtained by intersecting the surface with the planes $x=a$, $y=b$, and $z=c$. Try moving the mouse around and imagining how you could reconstruct the surface from these contour curves.