Funghini

The function

2 x y / (x^2+y^2) 

is discontinuous at (x,y)=(0,0). The graph is a surface of fixed height along each line through the origin - a very hard concept to wrap the mind around. This creates the fold at the middle. Note in the print below that there are four fixed height lines on it in a different color to emphasize this strange fact. The title of this post reflects the fact that the graph looks very much like a specialty pasta, though I am not completely sure Funghini is the right term.