The weight should be heavy enough to stretch the rubber band, but not so heavy that it is likely to break the band. Heat the rubber band with a hair dryer. Start the dryer and, when it has warmed up, turn its heat on the stretched rubber band. Doing an experiment several ways and checking for agreement in the results is an important strategy in science. When rubber is heated it behaves differently than most familiar materials.
Most materials expand when they are heated.
Consider the liquid in a thermometer. The thermometer works because the liquid expands when its temperature increases. Similarly, a wire made of metal, such as copper, becomes longer as it gets hotter. The expansion of metals with increasing temperature is the principle behind the functioning of home thermostats and of jumping discs. Whether a material expands or contracts when it is heated can be ascribed to a property of the material called its entropy.