system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is wanting to chew ice a sign of anemia? Why do you wanna chew ice if you have an iron deficiency?
gpt
I don't believe there is anything humans used to eat, that was ice-like, that ever gave us iron.

Pica is a mechanism to eat more diverse food sources, as a means to get the nutrients needed to survive that are missing from a diet, as a last ditch effort, by eating things that an animal doesn't consider edible. Normally this would be a good way to get sick, but when malnourished this might be a risk worth taking.

For example, lots of animals have odd dietary habits like licking rocks, for salt intake. Pica could be seen as a biological mechanism to get a goat who doesn't think rocks are food, to start licking them again. The same underlying forces are probably at work with humans, too, when they have a vitamin deficiency.