Challenge 1: Unwind a Tags Array — Possible Solution
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db.articles.aggregate([
  { $unwind: "$tags" },
  { $group: {
      _id: "$tags",
      article_count: { $sum: 1 }
  } },
  { $sort: { article_count: -1 } }
])

WHY THIS WORKS AS AN ANSWER
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$unwind: "$tags" runs first. An article with tags: ["mongodb",
"database", "nosql"] becomes THREE separate documents in the pipeline
— one with tags: "mongodb", one with tags: "database", one with
tags: "nosql" — each otherwise identical to the original article.

Only after that unwind does $group make sense: it groups by _id:
"$tags", which now refers to a single tag STRING per document (not an
array), since $unwind already flattened it. { $sum: 1 } counts how
many of those per-tag documents exist for each tag — equivalent to
counting how many articles used that tag, since each article
contributed exactly one document per tag it had.

The order matters: grouping by "$tags" BEFORE unwinding would try to
group by the whole array as a single value, so two articles with the
exact same three tags in the exact same order would group together,
but "mongodb" alone would never be counted as its own key.