system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some elevators have an up and a down button on their door, when they call the elevator to your floor regardless?
gpt
It calls an elevator that is already heading in that direction.

If you are on floor 5 of a 10 story building, you hit the _down_ button, and there is an elevator _already_ heading down, then the elevator will stop on your floor; if the elevator is heading _up_, it won't stop on your floor.  If the elevator doesn't have any passengers, then it will head up/down to you.