system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are objects in my car mirror closer than they appear? What's the point of that?
gpt
if your side-view mirror was a traditional flat mirror, you wouldn't be able to see very much because of its small size and distance from your eyes.  Instead, a convex mirror is used, which means it is curved.  This allows you to see a greater field of view, showing you more of the cars that are off to your side and behind you, so that you don't have a "blind spot" any more.  However, while this type of mirrow allows you to see more things, it makes them appear to be smaller.  When objects appear smaller, your brain thinks they are farther away then they actually are. 

The warning is there to remind you that they are closer than they appear and that you shouldn't change lanes or pull out in front of somebody based solely on what you think you see in the mirrow, instead, you should turn your head and glance at them quickly to make sure it's safe to do so.