usually if it's an basic good, or very important one.
because the price fluctuations do not affect the quantity sold.
a good example of that would be milk, if the milk gallon is say $8, and a family needs 1 gallon daily, they buy it for $8.
if the price drops to $7, they might buy 2, but they only need 1 everyday, just in case they may get another.
if the price drops to $4 or even $3, they're not going to get 10 gallons, there's no need for it on an everyday basis, besides is a perishable.
now if the price goes up to $12, they still need it, and will buy it for $12.