How can you prove that 13 is an unlucky number?

There is no actual way to prove 13 is an unlucky number. It's just by the storys of 13. You could just wait till Friday the thirteenth. That's the day when Jaque De Molay, the Grand Master of the Templar Knights, was supposedly burned at the stake by the Pope and French King Phillip. It is supposedly when Jaque comes back from the underworld and terrorizes us humans.