CMYK: Cyan, Yellow, Magenta, Black (don't ask me why "K" = black)
It's the standard 4-colour processing system used in most colour printmaking - colour newspapers, magazines, whatever. When designing for print in, say, photoshop, you have the option to switch from RGB (red, green, blue) to CYMK to better visualize on screen what the image will look like on paper.
Another example I would bring up is that when I was 12 or so, I put my Mom's glasses on, just goofing around, and noticed that through them the world actually looked a lot clearer. I didn't know I needed glasses before that - I assumed everybody saw things that were far away as being blurry.
(SURGEON GENERAL'S WARNING: The wearing of glasses not prescribed to you is not recommended, please do not try the above at home)