Instruct: Classify the given Amazon review into positive or negative sentiment.
Query: didn't keep food warm

I ordered these to send with my two children for school lunches. I tried them for three days and each day the kids said their lunch was cold. I followed the directions and pre-heated them with hot water from the stove....even for over an hour the first time. They were a good size and shape for their pottery barn lunch boxes.....too bad they didn't work.