system
Given the following post, respond as if you're commenting on Reddit:
human
How did fish get into isolated bodies of water such as ponds?
gpt
Many ponds were formed after rivers meandered away, often leaving the classic oxbow.  Even kettle lakes, formed by glacial retreat, were connected before isolation.  Other rare ways include weather phenomena, such as tornados, sucking up a pond here and spreading its contents all over.  Some fish travel short distances over land; not just walking catfish, also snakeheads.  Animals besides humans could also inadvertently catch a fish and drop it somewhere.

Nature finds a way.