Auto Mechanics to Quantum Mechanics

I have a schtick I do at the beginning of each class where I let the students ask any question, auto mechanics to quantum mechanics, we'll answer any questions you've got. Some former students asked me to revive it on-line, so occasionally I bring it out to the Playground and it's that time again. Leave any question or responses to questions in the comments and I'll get to as many as possible in this week's posts.

This time, however, I've got a couple questions for you, things that have been bugging me and I thought I'd throw them out to the hive mind:

-- What part of speech is the word "yes"? (I'm not talking about its usage as an exclamation in the deli scene from When Harry Met Sally Here)

-- If you take light of any two primary colors (red, blue, or yellow) and shine it at the same spot you get the secondary colors (orange, purple, and and green). Shine all three primary colors on a spot, you get white and shine none of them, you get black. How do you make brown?

-- When we smell something unpleasant, we say "p.u." what does it stand for?

-- One I've been playing with for years. The word "raise" and "raze" are opposites. "We raised the barn" means we put it up and "We razed the barn" means we took it down. Are there other pairs of words that are both homonyms and antonyms?

I've shown you mine, let's see yours. Any questions?