A Google-a-Day Puzzle for August 14

Our good friends at Google run a daily puzzle challenge and asked us to help get them out to the geeky masses. Each day's puzzle will task your googling skills a little more, leading you to Google mastery. Each morning at 12:01 a.m. Eastern time you'll see a new puzzle, and the previous day's answer (in invisitext) posted here.

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SPOILER WARNING:
We leave the comments on so people can work together to find the answer. As such, if you want to figure it out all by yourself, DON'T READ THE COMMENTS!

Also, with the knowledge that because others may publish their answers before you do, if you want to be able to search for information without accidentally seeing the answer somewhere, you can use the Google-a-Day site's search tool, which will automatically filter out published answers, to give you a spoiler-free experience.

And now, without further ado, we give you...

TODAY'S PUZZLE:

What cannot happen to energy according to the law that’s expressed mathematically as delta U = Q – W?

YESTERDAY'S ANSWER (mouseover to see):

Search [“Let no man who is not a mathematician read”] to find Leonardo da Vinci. Search [Leonardo da Vinci writing style] to learn that he used “mirror writing.”