The parts of speech are like a cake.

A noun is the subject of the sentence. It is a person, place, thing or sometimes an idea. A sentence would not be complete if there were not a subject noun. A noun is the main part of the predicate of the sentence.
The noun of the cake would be the different flavors of the cake. This is because the flavors are the cake, but they are more specific.

A pronoun can replace the noun in a sentence. Without a pronoun or a subject noun in a sentence, the sentence would not be complete. A pronoun can function by itself in a noun phrase and refers back to someone or something in the sentence or to a participant in the sentence.
The pronoun of the cake would be the cake. This is because it is what the cake is, in a more general way than the more specific flavors of the cake.

An adjective is a word or phrase naming a quality that is added to or related to a noun to modify or describe it. An adjective is usually placed just before the word that it modifies.
The adjective of the cake would be the sprinkles. This is because the sprinkles show the color of the cake.

A verb is a part of speech that describes the action, state or occurrence. Without the verb in the sentence, the sentence would be incomplete. It is the main part of the predicate of the sentence.
The verb of the cake would be the candles, because the candles burn on the cake.

An adverb modifies or qualifies and adjective, verb or another adverb. An adverb express a relation of place, time, circumstance, manner, cause, degree, etc...
The adverb would be the event that the cake is at. This is because it would describe the circumstance of the cake.

An one of a class of words that are used before nouns or pronouns (object of the preposition) to form a phrase that modifies verbs, nouns, or adjectives. They provided additional information.
The preposition of the cake would be the plate that it is on, because the plate is where the cake is.

A conjunction is a word that is used to connect clauses or sentences or to put words together in the same clause. There are 3 types of conjunctions; subordination, coordination, an correlative.
The conjunction of the cake would be the icing. This is because the icing connects all of the layers of the cake together.

A word or remark that expresses or shows emotion or an exclamation, protest, or command.
The interjection of the cake would be the decorations, because they are what makes the cake pop and they are what show the "emotion" of the cake.