saddle
Slicing this 3D surface (as entered in the construction by default) in planes parallel to the coordinate axes gives 2D curves that are ______ or _____.
That's why this 3D surface is called a _____ _____.
Notice that the point at the origin on the surface is both a local minimum and local maximum, depending on the direction in which you slice the surface.
Drag the magenta point on the vertical z-axis to adjust the magenta cross section that is perpendicular to that z-axis.
Drag the red point and/or green point on the horizontal x- or y-axis to adjust the red cross section and/or green cross section that is perpendicular to those axes.