As others said, it's that the core is a ceramic honeycomb with small passages, and that core is coated with platinum and a couple other precious metals as catalysts. When the surface gets hot, unburned gases react with additional air to burn them fully. That said, you can save by buying an off-brand one rather than one made by the manufacturer. If your car is old enough to need a new converter, you really shouldn't be buying any parts from the manufacturer anymore.
