"In English, the plural of ninja can be either unchanged as ninja, reflecting the Japanese language's lack of grammatical number, or the regular English plural ninjas."
http://en.wikipedia.org/wiki/Ninja
Sorry, but they are both correct, per the reference you cited.

Comedy gold on all 4 panels! Now I want to do this with my kids - but they're too young and don't know what a ninja is (yet). Not quite the same ring to it if I tell them to be "really useful, just like Thomas!"

...Like the NINJA!

You need to teach a class called, "How to insert pure win into everyday conversation."

@Rob and Invisible Editor on first page: "ninja" isn't even a Japanese word, anyway, it's the Chinese version of the written word. Not that Rob's point doesn't stand, I just like to correct people.

@Cat Ballou

It honestly didn't occur to me that she might have been asking 'Ninjas do chores?'

On reflection, I agree with you that it would have been a very natural reaction as her 'voice' isn't usually as omniscient as Scott's voice.

Thanks for making me think.

"And if we were speaking Japanese, that would mean something." - Invisible Editor

We're kind of speaking Japanese, as ninja is a Japanese word.

This reminds me of a pair of parents who took shameless advantage of their kid's dream. He wanted to be a baseball player: "Baseball players have to eat their vegetables" "Baseball players need to be able to focus, so focus on your homework" "You'll need to learn English because... all the teams have foreigners on them!" "You should focus on math and physics if you want to be able to calculate the trajectory of a pitch"