Challenge 1: Extending the Sentence Grammar With mouse/saw — Possible Solution
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sentence_demo.pl:

sentence --> noun_phrase, verb_phrase.
noun_phrase --> [the], noun.
verb_phrase --> verb, noun_phrase.
noun --> [cat].
noun --> [dog].
noun --> [mouse].
verb --> [chased].
verb --> [saw].

Query:

    ?- phrase(sentence, [the, cat, saw, the, mouse]).
    true.

    ?- phrase(sentence, [the, mouse, chased, the, dog]).
    true.

Explanation:
Adding noun --> [mouse]. and verb --> [saw]. simply gives noun/1 and
verb/1 a third and second alternative respectively, exactly the same
way an ordinary Prolog predicate gains a new clause -- nothing about
sentence, noun_phrase, or verb_phrase needed to change at all, since
they only ever call noun and verb, never enumerate the specific words
themselves. Both new sentences succeed because each individual word
now has a matching grammar rule, and the overall sentence structure
(the + noun, verb, the + noun) is satisfied by both new combinations
exactly as it was by the original cat/dog/chased words.

WHY THIS WORKS AS AN ANSWER
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This adds new vocabulary purely by adding new noun/verb alternatives,
demonstrating that DCG rules compose the same way ordinary Prolog
clauses do -- the higher-level grammar rules needed no changes at all
to accept the newly introduced words.