Each Joulie is a small metal shell filled with a phase-change material that melts at 140º Fahrenheit. (Some people might prefer 155º -- perhaps there will be a version for us.) So the Joulies absorb heat from the coffee till they reach 140º, and then start emitting heat. The effect (if you put in enough Joulies for your volume of beverage) is to maintain the coffee at the desired temperature for long enough to savor it. Using an insulated cup is recommended also.