The Word.A.Day list to which I'm subscribed has archaic words as this week's theme. The words for Wednesday, Thursday, and Friday are anon, gainsay, and harken. I think I've used all of those words at some point; I'm sure I've used the gainsay and hark, at least.

I figured out the fluids problem. I made one mistake very early on that led me to rule out solutions of a particular form, and was then immensely frustrated when I found every attack of even modest success seemed to drive me toward solutions of the form I'd ruled out as impossible.