Fascinating and wonderfully written article about the phenomenon of geophagy, or 'dirt eating', which nearly all animals - us included - partake in at one time or other.
There are various immunological reasons why children do it, and pregnant mothers have also been known to take a nip of soil as their bodies need the calcium to build the child's skeleton. Of course, it can also give you nasty things like guinea worm...
Some parrots in the Amazon manage to survive on a poisonous diet (of a particular plant), but only if they regularly visit a river bank and eat the poison-absorbing clay that can be found there.
I like the church angle in this piece too - that people ascibe miraculous powers to eating the dirt at this church. And they're right too (albeit for the wrong reasons, bless 'em).
Maybe some kind of question like:
Question: How should you respond if someone invites you to EAT DIRT?
Answer: Say 'Ooh, thanks awfully.'