Why do edible oils show rancidity when stored for long period?

Explanation

Rancidity occurs due to the oxidation of oils and fats which results in foul smell and change in taste.
ii. When edible oil or ghee is left aside for a long period without use, it reacts with atmospheric oxygen, gets oxidized and becomes rancid i.e., unfit for use. If food is cooked in such oil, its taste changes.