The most frustrating problems are those whose understanding is insufficient to solve them. They are the ones one must deal with for a prolonged period, and even if their mechanism is perfectly understandable, even if their solution is crystal clear, one cannot overcome them because one lacks the means to do so. Hence, each time they arise, it feels like a blow to the mind, a painful, exasperating blow that must be endured in silence, since reacting is futile and only increases the irritation. The worst part of such problems is that they seem unjustifiable the moment after they are understood; that is to say, what needed to be taught has already been taught; yet even so, out of sheer stubbornness, they insist on not going away.