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The phrase "objects in (the) mirror are closer than they appear" is a safety warning that is required to be engraved on passenger side mirrors of motor vehicles in the USA, Canada and India. It is present because while these mirrors' convexity gives them a useful field of view, it also makes objects appear smaller. Since smaller-appearing objects seem farther away than they actually are, a driver might make a maneuver such as a lane change assuming an adjacent vehicle is a safe distance behind, when in fact it is quite a bit closer. The warning serves as a reminder to the driver of this potential problem.
Despite its origin as a utilitarian safety warning, the phrase has become a well known catch phrase that has been used for many other purposes. Some of them are: