Well, a watermelon has a much higher specific heat than a sandwich. This
is because Water (H2O) has an unusually high specific heat, and the
watermelon contains much more water than the sandwich. The higher the
specific heat of an object the more calories it takes to increase its
temperature, this applies to cooling as well. So, a watermelon will take
longer to get cold, but it will stay cool longer.
Another factor is the surface area in comparison to each objects
mass. The Watermelon, in a sense, insulates the temperature of the