Definitions are the mind's comfort food. Why bother going on a rigorous diet of learning in detail how things work when you can pluck a definition out of a dictionary and ride with it? It's a rhetorical question, definitions never solve the immense task of explaining the world and its various bits to a 6-year old. Unless the knowledge has been ingurgitated and digested properly, until it's become part of oneself, there is no way any child would accept a packaged explanation. Is that why the best sites on astronomy topics are those tailored for kids?