Mine have taken to arguing, which devolves into crying, over abstract concepts. For no apparent reason, the 4 year-old will just lean over to the just 3 year-old and say, 'You can't have red.' Not any actual red object, just the colour red.
Of course, having been told he can't have the abstract concept of 'red', it's now the thing he wants most in the world, and lets out an enormous wail insisting that he must now have red. Of course, once he's crying, the one who started it gets upset and joins in.
So I'm left with two wailing kids, trying to work out how to settle an argument where nothing is actually being argued over.