You, reading Skullanator in 1998, would abandon your idea of a guy with a hammer running through my junior high. Matt could go on using the idea and even sue me for my Skullanator profits. He would be the original author. It is important not only to prove that you came up with the idea but that you did so before anyone else. There are several ways to do that.
Publication is the most common manner.
A simple trick is to mail oneself a copy of the work, the cancellation date of the stamp being the earliest copyright date. This is a very bad idea. Far too many things can go wrong. If you do this trick, at least remember to place the stamp along the seal of the envelope to prove you didn't mail yourself an empty envelope in 1988 and stick a manuscript in it ten years later. The Writer's Guild of America actually has a high tech version of the canceled stamp trick in which they, for a fee, date stamp and warehouse a theatrical manuscript for you.