Our children are the canary in the coal mine of marijuana legalization.
Coal mines were/are dangerous places. Fumes can leak in undetected. Mining tragedies were not uncommon. Many miners were killed in explosions, asphyxiations or poisonings before they were aware that a hazardous substance had leaked into their midst.
But a tiny bird became their warning signal. A canary in a cage in the mine shaft, with its delicate constitution, would succumb to the hazard long before the men would sense it was there. When the canary showed signs of illnes, the miners new it was time to get out.