Smoking point is referred to in the heating of cooking oils as an indicator of an oil's ability to withstand high temperature. At the smoking point, an oil begins to break down, emitting a gas and various byproducts, some of which contribute off-flavors to the oil. Oils with higher smoking points are desirable for deep fat frying, such as peanut oil and corn oil. Fats which have been previously used for deep frying have a lower smoking point the second time because the breakdown has already begun. Mustard oil is the exception to the rule in terms of a flavor change after the smoking point is reached. Mustard oil achieves a more desirable flavor after heating to the smoking point.