Challenge 1: A Query That Sees a Later Addition — Possible Solution
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Program.cs:

var numbers = new List<int> { 1, 3, 6, 8 };
var query = numbers.Where(n => n > 5);   // NOT executed yet

numbers.Add(20);   // added AFTER the query was defined

foreach (var n in query)
{
    Console.WriteLine(n);
}

Output:

    6
    8
    20

Explanation:
`query` doesn't run when it's assigned -- it only describes the
Where() operation. The actual filtering happens when the foreach loop
enumerates it, which occurs AFTER numbers.Add(20) has already run. By
that point 20 is genuinely part of the source list, so it's included
in the filtered result exactly like any other qualifying element,
even though it didn't exist when query was first written.

WHY THIS WORKS AS AN ANSWER
------------------------------
This reproduces the chapter's own deferred-execution example, showing
a later mutation to the source collection is picked up by the query
because nothing runs until enumeration time.