sentences: a class of something means a type of something & classifying is when one sorts something by kind / type ->  classifying is when one sorts something by class  classifying is when one sorts something by class & class is a measure of how a rock is formed ->  rocks can be classified based on how they were formed  rocks can be classified based on how they were formed & igneous rocks are formed by magma cooling / crystallizing -> a rock can be classified as igneous if it was formed from magma