Keep Car Windows from Fogging w/a Potato 1. Cut a raw potato in half. Be sure to use a clean potato, so wash it first if it is coated in soil. This is a good opportunity to use up an old potato that has started to sprout and is past its best. 2. Rub the cut part of one half (use second half if needed)potato on the window. This will clean it and leave a layer that will prevent fog from forming on the window. 3. Leave to dry without touching.