Put food to the edge of the plate in a microwave oven

When a microwave oven is in operation the space in the microwave contains nodes and anti-nodes. Food at nodes will be heated more quickly than average, food at anti-nodes will stay cold.

If you place food in the center of the rotating plate, it will not move much. The center may be stuck in a node or antinode and will be heated differently to the rest. To heat food evenly, place it as close to the edge of the rotating plate as possible.