That there exists an explanation does not imply that the explanation is obvious.
I understand that light may act as a wave or particle depending on how you study it. However, this fact is not obvious. It is in fact incredibly non-obvious, which is how it took science so long to get there.
That [caller(0)]-> works is (at least to me) obvious. That (caller(0)) works and caller(0) fails is (again, to me) non-obvious.