When x=0, no matter what y is, z is 0. When y=0, no matter what X is, Z is 0. Then when x=y, z=x*x=y*y, then you can see an image, which is the same as when the curve of y=x*x makes a 45-degree angle along the X axis or Y axis on the plane, and this is where the maximum value lies. When x*y=- 1, the opposite is true. Then you can get saddle-shaped graphics through spatial imagination.