Could we also use "kitty" to a human?

As an adverb, what does "solemnly" mean? In what circumstances could I use it?

As an adverb, "solemnly" means with deep sincerity or in a formal manner. It's often used in legal matters when you "solemnly swear" something - to swear to say or do something with deep sincerity. Or if you're just doing something with sincerity. It can also reference official matters. Ex: He solemnly vowed to come back to the city next year to visit. Ex: She was solemnly sworn into the government office that week.

How is it different between "I can only think about her" and "I can't stop thinking about her"? Is there any difference in nuance?

Yes, there is a difference between "I can only think about her" and "I can't stop thinking about her". If you say "I can't stop thinking about her", it implies that you are trying to stop thinking about her but you are unable to stop. "I can only think about her" implies that you want to think about her and you aren't trying to stop. The expression "I can't stop thinking about her" is also more common to use.

What does "windbag" mean?

"Windbag" is a noun that refers to a person who speaks a lot, but is not saying anything important. Calling someone a "windbag" is informal and an insult. Ex: That politician is a windbag, all he does is talk, but he never takes action.

What is the difference between "race" and "wait"?

"Race" means to compete with someone to see who is the fastest, whereas "wait" means to stay where one is to be patient until something happens. Ex: I'll wait for you at the restaurant. => the person is at the restaurant first, waiting for the other to arrive. Ex: My friends and I race on our bikes. I usually win.

Just curious, what's the origin of the currency "dollar?" Where does the name come from?

The word dollar actually comes from the early German word "Thaler," a silver coin in the Roman Empire in the 16th century. The root word for "thaler" in German is "thal," meaning valley. So a "thaler" is a person or thing from the valley. Ex: The price is one thaler. Ex: The word "dollar" is the anglicized version of the German word "thaler."