The specific gravity of a substance is a comparison of its density to that of water. Imagine a gallon bottle filled with water, a second filled with feathers, a third filled with lead weights. There are equal volumes of material present, but the bottle with the feathers will weigh less than that containing water; the bottle with lead weights will weigh the most. In order of increasing specific gravity, these materials would be: feathers, water, lead. Specific gravity can be measured precisely, or estimated by a comparison, as above. To compare the specific gravity of any two minerals, simply hold a sample of one in your hand and "heft it," i.e., get a feeling for its weight. Then heft a sample of the other that is approximately the same size. If there is a great difference in specific gravity, you will detect it easily. It is often sufficient to note whether a mineral's specific gravity is significantly higher or lower than that of other minerals. Heft each of the specimens in your mineral set. Which ones have a high specific gravity? A low specific gravity?