Challenge 3: ToList() Freezes the Result at Materialization Time — Possible Solution
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Program.cs:

var numbers = new List<int> { 1, 3, 6, 8 };
var snapshot = numbers.Where(n => n > 5).ToList();   // executed IMMEDIATELY, right here

numbers.Add(20);   // added AFTER the snapshot was already materialized

Console.WriteLine(string.Join(", ", snapshot));

Output:

    6, 8

Explanation:
Unlike Challenge 1's deferred query, .ToList() forces the Where()
filter to run immediately, right where it's called, producing a real,
independent List<int> -- snapshot -- that has no ongoing connection to
the original numbers list at all. Adding 20 to numbers afterward has
no effect on snapshot, since snapshot is no longer a description of a
query; it's already a finished, materialized result frozen at the
moment ToList() ran.

WHY THIS WORKS AS AN ANSWER
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This directly contrasts with Challenge 1 by adding .ToList(), showing
the exact same source-mutation-after-query-definition scenario now
produces a stable result unaffected by the later change, matching the
chapter's own tip-box guidance on forcing immediate evaluation.