Very well, I begin to cipher out the German for that answer. I begin at the wrong end, necessarily, for that is the German idea. I say to myself, "Regen (rain) is masculine -- or maybe it is feminine -- or possibly neuter -- it is too much trouble to look now. Therefore, it is either der (the) Regen, or die (the) Regen, or das (the) Regen, according to which gender it may turn out to be when I look. In the interest of science, I will cipher it out on the hypothesis that it is masculine. Very well -- then the rain is der Regen, if it is simply in the quiescent state of being mentioned, without enlargement or discussion -- Nominative case; but if this rain is lying around, in a kind of a general way on the ground, it is then definitely located, it is doing something -- that is, resting (which is one of the German grammar's ideas of doing something), and this throws the rain into the Dative case, and makes it dem Regen. However, this rain is not resting, but is doing something actively, -- it is falling -- to interfere with the bird, likely -- and this indicates movement, which has the effect of sliding it into the Accusative case and changing dem Regen into den Regen." Having completed the grammatical horoscope of this matter, I answer up confidently and state in German that the bird is staying in the blacksmith shop "wegen (on account of) den Regen." Then the teacher lets me softly down with the remark that whenever the word "wegen" drops into a sentence, it always throws that subject into the Genitive case, regardless of consequences -- and that therefore this bird stayed in the blacksmith shop "wegen des Regens."
N. B. -- I was informed, later, by a higher authority, that there was an "exception" which permits one to say "wegen dem Regen" in certain peculiar and complex circumstances, but that this exception is not extended to anything but rain.