2006I ignored the question of whether "fall(ing) out" is an idiom or not. Interestingly, "fall out" is listed on this site as a phrasal verb but not as an idiom. It is both a phrasal verb with a literal meaning and an idiom, as Munch indicated..
I think that if students are interested in idioms, and some clearly are, they would be interested in knowing what the definition of an idiom is.I fear that we may be heading towards a discussion on what precisely we mean by 'idiom', which may be fascinating for those taking part, but of little value to most students.
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