This is an interesting experiment! One idea that comes to my mind is conducting your experiment with different types of nuts e.g. peanuts, cashew, etc and see which one is able to elevate the temperature of the water the most (releasing the largest amount of calories). So in this case, your independent variable would be the type of nuts and your dependent variable would be the temperature of the water.
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http://www.sciencebuddies.org/mentoring ... bles.shtml