One good method for reheating French fries is to use a frying pan on the stove top. Fries will typically already have enough oil in them for recooking, but oil can be added if necessary. The fries should be tossed gently until they start to sizzle.
Another effective method for reheating fries is in an oven that has been preheated to approximately 450 degrees. The fries can be placed on a baking sheet and cooked for 2 to 3 minutes to heat them all the way through.
A microwave should never be used to reheat French fries. This appliance will make leftover fries soggy and inedible.