For the purpose of an informative piece (be it an article or essay), you will have a very distinct goal of teaching your audience or readers, by manner of presenting information (demonstrating), how to do something or give them enough information by manner of defining or describing something to where they could do something on their own.
He first used them on Dudley's birthday when he made the glass disappear then reappear causing Dudley to fall in the snake enclosure and stay trapped inside while the snake escaped.
