Challenge 1: A Sorted, Deduplicated Category List — Solution Walkthrough

Data: A1:A12 = Groceries, Transport, Groceries, Entertainment,
Transport, Groceries, Utilities, Transport, Entertainment, Groceries,
Rent, Transport.

Formula (typed into C1 only):
  =SORT(UNIQUE(A1:A12))

What spills, and where:

UNIQUE(A1:A12) first reduces the 12 entries down to their 5 distinct
values, in first-seen order: Groceries, Transport, Entertainment,
Utilities, Rent.

SORT then reorders that 5-value result alphabetically (ascending is
the default when no sort order is specified):
  C1: Entertainment
  C2: Groceries
  C3: Rent
  C4: Transport
  C5: Utilities

The final result spills into C1:C5 — a single column, 5 rows, from
one formula typed only into C1.

Explanation:

This is the exact nested pattern this chapter introduces: UNIQUE's
spilled array becomes SORT's input directly, with no intermediate
helper column needed. Note the formula was typed ONCE, into C1 —
attempting to also type something into C2:C5 (thinking they need
filling in individually, the way Chapter 2's fill handle works) would
actually block the spill and produce a #SPILL! error instead, per
this chapter's own warning box.

Notes:
- If two more "Groceries" rows were added to A1:A12 tomorrow, this
  same C1 formula would still show exactly 5 rows (Groceries is
  already in the unique list) — only a genuinely NEW category
  appearing in column A would grow the spill range to 6 rows.