The following post has been brought to you by the letter N, the number Death, and 40 billion ninja. I Look Forward to Killing You Soon - Hiya! The rest of this post is in black in honour of the day.
The Day of the Ninja on December 5 is like Talk Like a Pirate Day... except that ninja do not talk like idiots... or at all, in fact. Just sweet, festive, deadly silence. And they have both eyes. Sometimes more than both.
The biggest secret of the ninja is their stealth. There could be as many as four hundred ninja hiding in the full stop at the end of this sentence. Ahah, you're not dead! That's either because there aren't any ninja hiding there at this exact moment or because they simply choose not to kill you right now.
The stereotypical ninja that continually wears easily identifiable black outfits (shinobi shozoku) comes from Kabuki theatre. Prop handlers would dress in black and move props around on the stage. The audience would obviously see the prop handlers, but would pretend they were invisible. Building on that willing suspension of disbelief, ninja characters also came to be portrayed in the theatre as wearing similar all-black suits.
This either implied to the audience that the ninja were also invisible, or simply made the audience unable to tell a ninja character from many prop handlers until the ninja character distinguished himself from the other stagehands with a scripted attack or assassination. If there were any actual ninja in the theatre then most likely they'd be hiding in the audience dressed as audience members, so that if anyone was mysteriously killed, there would be no obvious suspects.
As stated in Horse 566, Ninja are a fully unionised labour force (unlike Zombies) and would probably win against Pirates in a fight. Unlike Vampires and Pirates, Ninja are not a university trained profession, sometimes spending many years as an apprentice under a great master.
Because of this, the skills of the ninja are varied; sometimes engaging in crowd control, practising law, providing transportation, general assassination, medical procedures where their exacting skills with knives are able to cut away delicate organs, and also in the kitchen where yet again their skills with knives are impeccable. Have you ever seen ninja prepared Radish Rosettes or Fugu? Those are very hard to make without killing someone... er... stay away from the Fugu.
If you can see a Ninja, he is not a Ninja. If you can't see a Ninja you're probably going to die very quickly. Ninja can live in your house for months without you noticing. Ninja can kill people With all sort of things, forks, Gnomes and Music. (Bjork is not a ninja, this has been proven by scientists.)
Ninja have also known to branch into other areas like the Secret Urban Exploration Ninja Mafia who sneak into illegal territory. The regular Ninja Mafia are different however and as an underground organistation (both figuratively and literally) have their own rather ironically named High Council. The Black Death which swept Europe in the 14th and 15th Centuries was actually carried out by the Ninja Mafia using secret techniques. Before the Ninja Mafia came along, it was just known as "that thing that's going around". Can Ninjas catch cold? Absolutely, but it's much the same way as I can catch a cricket ball - they can literally hold a cold in the palm of their hand then slice apart the individual germs. Can colds catch ninjas? Nope - They're faster than germs; in fact the only disease that a ninja is susceptible to is Saturday Night Fever.
Finally, if you ever hear voices in your head, it is because a ninja has snuck in and is talking to you. Beware!
Yet again we are all astounded by the wisdom of Mr Mandela.