This file helps to understand the lecture 6 - DiffGeom - from Prof. Wildberger. (https://www.youtube.com/watch?v=SSacdJZXCXc&index=30&list=PLIljB45xT85DWUiFYYGqJVtfnkUFWkKtP)
When we change the Level variable, we can observe changes of curves of level.
When we change the point A, we can observe changes of tangent surfaces - plane and quadric.