If one of the AC knobs in your vehicle falls off, you’d probably want to take it back to the dealer and get a genuine replacement part. That, however, might cost you a pretty penny, and that’s also assuming that the dealer has such random spare parts on hand. You could go to your local scrapyard and hope for the best, or you could do the smart thing and purchase an inexpensive spout knob.
What this Ph.D. holder in DIY engineering did was use a regular spout knob to control the car’s AC airflow direction. If one of the other knobs falls off, this vehicle owner knows exactly what to do.