If you think it's not a lame mathematical trick, would you care to say how you think it's done? Or at least, say something that makes it clear to those who know how it's done that you also know the answer? Like, for example, tell us how it can go wrong.

Although I'm not actually involved in this event, it is the sort of thing that I follow closely, and on other occasions am involved in. I am especially interested to discover what people already know so I can avoid well-known, obvious or boring stuff, and produce things that are more challenging.