Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

Q: How should you reply to 'what's up?'.

A: 'What's up' means 'What's happening.' I usually just reply 'nothing.' because nothing is happening to me. But, there are alternatives, such as the usual reply to a greeting: Not much Nothing. Yourself? I'm good, yourself? Good. How are you? However, if there is something that you would like to tell the other person e.g. You've sold your car, it might go like this: A: What's up? B: I've sold my car. A: Good on you mate!
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Q: A word for the unexpected reaction of laughing in a situation of complete hopelessness or laughing with sad undertones.

A: Gallows humor is 'humor that treats serious, frightening, or painful subject matter in a light or satirical way.' It has the connotation of someone laughing at their own execution. Urban Dictionary actually has a good summary: gallows humor Sense of humor that arises from stressful, traumatic or life-threatening situations where death seems to be an inevitability. Different from black humor in that comments of this sort are made by the person/persons affected (i.e. about to die). It is not always used to mean that the person is facing literal death, but the idea is the same. They are expressing amusement or using humor in the face of their own inevitable fate.
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Q: It seems that exemption and exception have the same meaning. However, they seem to be used differently. When should we use one and not the other?.

A:
They do have similar meanings. They both refer to leaving out an item from a set. The difference is how they are left out. an exemption is left out of the set by an authority or man-made rule. For example, a tax exemption, is money you received but does not have to be counted as part of our income because the tax laws explicitly say it's OK to ignore (presumably with the law created for some economic incentive purpose). an exception has no such man-made connotations (i.e. an exemption might be considered a kind of exception, but might be totally arbitrary). The number 2 is an exception to the idea that all prime numbers are odd (one would never use exemption in that context).
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