3 ideas
The series divides knowledge into self-contained volumes, each devoted to a single subject such as volcanoes, the Middle Ages or electricity, rather than one alphabetical encyclopaedia. A child can pick up whichever topic currently interests them and get a complete, bounded treatment without first learning the whole reference apparatus. Each finished volume also marks a visible unit of mastery, and the collection grows one interest at a time.
The early volumes were checked by academic specialists before publication. This keeps simplified text for children accurate and gives parents and teachers a reason to trust the books. Simplification is treated as a job for experts, because explaining something in plain language without distorting it requires knowing which details can be dropped safely.
The series title means 'What is what', and it presents the natural and human world as a set of questions a child can ask and get answered. Topics are approached through what a young reader would wonder about, such as how something works, why it happens or where it came from. This makes the reader's curiosity the entry point and frames not knowing as the start of inquiry rather than a failure.