Teacher: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.
Teacher: Now, understand the problem? Solve this instance: I've found that expression on a website, and I don't understand it. At first I thought it was because the canary is yellow and the coal black... but it doesn't make much sense.
Student:
A canary in a coal mine is an advanced warning of some danger. The metaphor originates from the times when miners used to carry caged canaries while at work; if there was any methane or carbon monoxide in the mine, the canary would die before the levels of the gas reached those hazardous to humans. See here.