In the third phase, differentiation, children begin to learn that written language regularly differs in structure and style from spoken language. The growth from consolidation to differentiation can be challenging for some children to grasp. Children can 'struggle with the transformation from the basically overt language of speech to the essentially covert activity of writing'. In this phase, the child learns that writing is generally considered more formal than spoken language, which is thought to be casual and conversational. Here, it is believed that children begin to understand that writing serves a purpose.