Hyperbolic Slot

HYPERBOLIC SLOT displays an aspect of mathematics which is positively arresting. A flat vertical plane made of plastic is mounted on a table top; there is a curve cut into the plastic sheet. Next to the plastic sheet stands a straight stick which is mounted on a vertical rod. The stick, even though it is straight, swings cleanly through the radically curved slot in the plastic. Graphics explain why this is so, and describe the nature of the curve.