You open a grayscale image in PhotoShop, Select all, Copy, close the file, Open a new file......PhotoShop automatically sets the new file as a grayscale even if your intial default settings are for RGB. Now anything you paste into that particular file will convert to GS. This usually happens because we are working..."on the fly". I've created a habit of looking at the new file Panel each time I create a new file, and each time I save the file to note the mode. Cool Entry!

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Ockham's razor- It's that simple