You plan to marinate a piece of beef for your dinner. The beef is 5 by 5 inches with the thickness of 1 inch. You rub salt and pepper all around it, and set it aside for 30 minutes at room teperature. Then you checked it after 30 minutes and found that the beef tastes salty at about 1/5 inch from the surface (Note that you still have not cooked it yet. Hope that you enjoy your raw beef at the moment).