Ellipses are conic sections.
Mathematical definition of an ellipse
An ellipse is the set of all points such that the sum of the distances of P from two fixed points F1 and F2 is constant. Each fixed point is called a focus (plural foci).
The ellipse has a reflective property...If a tangent line were drawn at point
P, the angles formed by that line and the lines PF' and PF would be equal. Thus
a light or sound wave emanating from one focus of an elliptic surface would
be reflected to the other focus...this is called a "whispering gallery."
A whisper at one focus is audible at the other. This famous architecture can
be found along the base of the dome of St.
Paul's Cathedral in London, built by Christopher Wren. Check out the whispering
gallery exhibit in Chicago's
Museum of Science and Industry.