system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have nightmares?
gpt
One interesting thing is that children are much more likely to have nightmares about monsters chasing them. As we grow up we start to substitute that fear to things in our world. But maybe we have nightmares to prepare ourselves for terrifying situations, so that when you're being chased by lion you've had a couple practice runs.