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Many people don't take the time to read their car's manual after buying a vehicle, so a majority of car owners don't know what every button in their automobile does, including one that they look at every time they drive. It's on the rear view mirror and has a power symbol on it, but many drivers just look at it and wonder why you would ever have to turn the mirror on or off.
One person took that inquiry to Reddit's "What Is This" forum where they asked about the button. Commenters chimed in to say they also had no idea what it did, including one who wrote, "The only thing that happens when I click the power button is the green light turns off."
Those in the know though quickly explained what the button does - it activates the mirror's auto-dimming feature, which only works at night.
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Per one Mazda dealer's site, "When auto-dimming is turned on, your rear-view mirror will automatically darken in order to reduce headlight glare. There is a rear light sensor that can detect when a vehicle behind you is shining its headlights toward your mirrors. Anytime the vehicle is started, the auto-dimming feature will be activated."
It should be noted that not every car has the feature, and those that do might have two buttons for it - one to turn it on and one to disable it.
As for the reactions from those learning about the auto-dimmer for the first time, one joyful person stated, "Another great life mystery solved," but others claimed the dimmer doesn't do anything.