In the early 20th century, coal miners would carry canaries into the mines to check for dangerous gases. if the canary got sick died, then it meant that poisonous gases were present in the mines, and the miners would either avoid working in that mine or use protective gear. the advent of modern technology and animal rights groups put an end to this practice, but the phrase "canary in the coal mine" is still used to mean a warning of danger to come. this practice is also the premise of the shooting game, canary.
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