How to prevent french fries from hardening after recooking...?

I baked my son some french fries in the oven, they were great. I froze them in a ziplock bag so he could have them along with another dinner the next night. I microwaved them and they were soggy, a few minutes later they were as hard as rocks!!! Is there a way to cook them so that they are soft like the first time i baked them?