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You may be tempted to heat your kittys leftover food in the microwave, but I don't recommend this.
My research says that cats should eat food that is room temperature. Any food that is too warm or too cold can cause intestinal troubles for your cat.
However, leaving a whole can of food out of the refrigerator will cause it to spoil if your room temperature is above 65F or so.
If you have a 'cold' window, you can store the open can of food on the window sill with a tight fitting cover. This will at least keep the food cold enough to prevent spoilage, but allow the food to heat up faster.
I heat a small amount of water (up to boiling) and then stir a couple of spoonfulls into the cold food. This will bring the temperature of the cold food up to something more satisfactory for your cat, without getting the food too hot.
Note: You don't want to add too much water, or your cat or kitten won't care for the food.