Sweat of the brow
I have always understood that in the US, mere sweat of the brow does not attach copyrights to a creation. In order to be copyrighted, a work needs to be original. This is important for reproductions of public domain works, for instance photos of public domain paintings. I also understood that this is specific to the US, and that in Europe different, less enlightened rules apply.
My first understanding stems from a ruling in the US, Bridgeman vs. Corel, in which the judge finds that a work needs to be original to generate a copyright.