  When we say "I know why he was late", we know not only the fact that he was
late, but also an explanation of this fact. We propose a logical framework of
"knowing why" inspired by the existing formal studies on why-questions,
scientific explanation, and justification logic. We introduce the Ky_i operator
into the language of epistemic logic to express "agent i knows why phi" and
propose a Kripke-style semantics of such expressions in terms of knowing an
explanation of phi. We obtain two sound and complete axiomatizations w.r.t. two
different model classes depending on different assumptions about introspection.
