I gathered some facts about them:
Ninja don't sweat.
Bullets can't kill a ninja.
Ninja invented skateboarding
Only a ninja can kill a ninja. Regular humans are useless.
Ninja never wear headbands with the word "ninja" printed on them.
Ninja can breath underwater anytime they want.
Ninja can change clothes in less than 1 second.
Ninja don't smoke, but they do use smoke bombs.
Ninja always land on their feet. If they don't have feet they will land on their nubs.
Ninja invented the internet.
Ninja don't eat or drink very much, and they never have to go to the bathroom.
Ninja always move to America when making a new start as a non-assassin.
Ninja don't play sports. Unless killing is a sport.
Ninja can crush golfballs with 2 fingers, any two fingers.
Ninja have a bad temper when they lose at anything. They will usually cut off the winners head before they have time to gloat.
Ninja lie all the time. Even when the truth serves better, ninja will lie anyway.
Ninja swords are always straight with a square handle guard. Always. Curves are for girls.
Lack any personality
Fight skillfully with any object
Can remove a spleen in one swift motion
Live in your house secretly for days
Can remove their shadow if needed
Go anywhere they want instantly
Catch bullets in their teeth
Kill themselves if they make a noise
Can run 100 miles on their hands
Train 20 hours/day starting from age 2
Have cool words like Seppuku
Are masters of disguise
Can hover for hours
Flip out and kill everything
Are completely self-sufficient.
Split planks vertically with their nose
Can hide in incense smoke
Ninjas are the best guitar players. Ever.
Ninjas do NOT wear spandex.
A Samurai is NOT a ninja.
Dragon Ball Z characters are NOT ninjas.
If you see a ninja, he is NOT a ninja.
Some guy: "Ninjas are totally sweet"
Some other guy: "True true"