system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do rainbows last so long? I know they come from the refraction of light through rain drops, but aren't those drops always moving? What keeps a rainbow stationary and constant?
gpt
It's the same way that cars crossing a bridge are always moving, but "traffic on the bridge" seems stationary and constant.  Some cars exit the bridge, new cars enter the bridge, and so you're looking at different cars, but the same traffic.