@Advocate,
What is there to explain, A? Things do not have to make sense to be
grammatical.
Colourless green ideas sleep furiously.
is an example (Chomsky) of a nonsense sentence that is fully grammatical.
Any part of speech can be the subject of a sentence.
Quickly is an adverb.
A is an article.
On functions as a preposition.
Impossible is a piece of cake.