"It is the everyday things"

Such a construction: "It is the everyday things..." - and the likes, baffle me.
Verb: singular - noun: plural
Of course I intuitionally understand, why. But could somebody please explain me the formal rule for that (if there is any ;) )??

Here, the speaker is grouping all of the things into a single category that will have a single effect, or whatever the sentence goes on to say. Singular and plural are often not straightforward mathematical choices.