If you want your leftover fries to be crispy like they were when you bought them, a microwave isn't the appliance to use. Microwaving fries makes them warm, but limp and soggy. If you don't mind this texture, simply place them on a microwave-safe plate, cover the plate loosely with a paper towel and microwave the fries on high-heat at 20-second intervals until warmed to your liking. Better ways to reheat fries exist, though.
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Microwaving encourages moisture to be released from food, so it works great when reheating soup, braises or other juicy foods -- but terribly when you're trying to recreate a crispy crust and soft center. The nuanced texture of fried foods, such as french fries, is beyond a microwave oven's capabilities.
A toaster oven or regular oven provides a superior way to reheat french fries or any other fried food. To use a toaster oven:
Step 1: Spread Out the Fries
Place the fries on a small metal baking tray and spread them out in one layer. Avoid crowding them on the tray.
Step 2: Preheat the Oven
Set the temperature to 350 F. Any hotter and the fries may burn before they heat through.
Step 3: Reheat
Slide the tray into the toaster oven and reheat the fries for 5 to 7 minutes. Check them every few minutes to see if they've reached the desired temperature.
Step 4: Remove
Remove the fries from the toaster oven and eat immediately. If they cool off, they'll turn soggy again.
Use the same method for reheating fries in a standard oven. If you have a large batch to heat, you can use a larger tray and do more at once. Still space out the fries to maximize crispiness. If they're too close together, it encourages steaming.