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.
Example: How are i.e. and e.g. pronounced?
Example solution: i.e. stands for id est (Latin), which means 'that is'. You use it to link in a deeper explanation about something. Pronounce it 'eye - ee'. e.g. stands for exempli gratia (also Latin), which means 'for example'. You use it to link in an example of a more generic term. Pronounce it 'ee - jee'
Example explanation: Perfect answer because it's consistent with context uses required information from the question to provide the detailed answer with an example.

Problem: As generally agreed and as extensively discussed in this question, 'an' should be used in place of the more common 'a' where the following word begins with a vowel sound. I have just encountered for the first time an instance of the phrasing an mule in the book Why Chemical Reactions Happen (James Keeler and Peter Wothers, Oxford University Press, 2003). (This book is not mine. I was sitting opposite a friend when the friend mentioned the unusual phrasing.) This phrasing appears to contradict the rule discussed above, since 'mule' is, as far as I am aware, pronounced /mjul/ (see Wiktionary). Here is the context. The author is using an analogy to explain a chemistry concept (the italics are not mine, but the bold emphasis is): A useful analogy here is that of an mule, which we get by crossing a horse with a donkey. To describe an mule as a horse or a donkey would be inaccurate and it certainly does not interconvert rapidly between the two! Rather we need to recognize that although an mule has parts that are reminiscent of a horse and parts that a reminiscent of a donkey, it is something altogether different. Looking for other examples online, I have managed to find one on Wikipedia's List of Siege engines. Discussing a Siege engine, the Scorpio: Similar to the ballista, but smaller. Was sometimes mounted on an mule-drawn cart. Other examples are difficult to find, but here's one from a user-posted question on Answers.com (referring, notably, to a kind of deer rather than the animal, a mule): Is an mule deer an omnivore? No. Mule deer are herbivores. ... What is going on? Is there an irregular pronunication of 'mule' of which I am not aware? Is this some strange language quirk, some archaic spelling that I am encountering for the first time?.
Solution: The original printing had 'an ass' instead of 'an mule'. That error was not lost on the chemistry community. See, for instance, Krenos's 2004 review of the text in the Journal of Chemical Education: In Chapter 10, bonding in extended conjugated systems and resonance are introduced with the horse + donkey = ass analogy (it is likely the hybrid animal mule is intended instead of ass, however). It seems the editors subsequently made an error mistake.