Deodorant Cap (AR Modeling Challenge)

This deodorant cap looks like a hyperbolic paraboloid. The equation of the blue surface used to graph this within GeoGebra Augmented Reality can be seen at the bottom of the screen. Note the restrictions placed on the domain of this surface function. Questions: 1) What is the equation of the pink plane (at the bottom)? 2) What are the domain restrictions of this surface equation? After answering questions (1) and (2), check the accuracy of your responses by graphing both surfaces in GeoGebra Augmented Reality! Other than a Pringles potato chip, what other every-day, real-life, 3D objects look like hyperbolic paraboloids?

Quick Demo