system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How snails form their shells

I have a pet snail. Is its shell built by the snail as it grows and needs more space? Or does it grow like a toe nail? How is it attached? 

Bonus question: how different are snails and slugs?
gpt
The shell is attached by the inner surface, and it grows a bit like a toenail. Snails and slugs are not exactly taxonomic terms, but in general they're actually pretty far apart. 

Assuming you're talking about just land ones, snails are generally herbivores while slugs will eat anything. Dead animals, insects, plants, mushrooms, worms, and all sorts of things will get eaten by slugs while snails will *usually* stay away from most of that. There are carnivorous land snails though and many water snails also eat meat.