Definition from me:<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
For very young schoolchildren, writing means constructing at least 4 or 5 logically related sentences around one central topic. For students at intermediate level of language, the simple piece should be expanded into 10-or-so sentences that maintain unity about one central topic and that are developed from at least 2 parts (subtopics).