The Canary Theory

For one of my classes that I am taking I'm reading a book entitled Raising Children in a Socially Toxic Environment. While the book is a pretty big downer, it gave an analogy that I found amazingily insightful.

During the 1950's coal miners would take Canaries in a cage with them down into the mine. These canaries would sing but canaries are particularly vulnerable to toxins such as methane. If toxin levels in the mine grew to a dangerous level, the canaries would die and the miners would then know to it was no longer safe.

Besides the point that this is brilliant on the miner's part I couldn't help but understand the beauty of the analogy.

What is your canary and has it stopped singing?