system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where do shells come from?
gpt
There are animals that lived in them at one point (or still do) and they create the shells around them out of minerals they consume the same way you grow hair. Sometimes those animals move onto bigger shells later, abandoning the small ones, and sometimes they stay in them till they die and the shells just keep getting bigger to accommodate them.