When dealing with the bulk qualities of different rock types the concept of density is often confused with whether an object is relatively light or heavy, which is merely its weight. In its simplest form the weight of a given volume of material can be compared to the same volume of water to get its specific gravity. This can be quoted without any units, unlike densities, which must be formally quoted with the relevant units of mass and volume. While this concept should be grasped fairly early on in secondary school, understanding density can be used to distinguish those who can deal with more complex scientific ideas. This is why manufacturers sell ice cream by volume, rather than weight, as the more air they whip into their products the lighter they become and for a given volume the less they will have to sell, which makes comparisons between brands almost impossible without tasting them! Unfortunately, many commentators can find it difficult to make clear the distinction between the heaviness of an object and its density.
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