Why is a LINQ query called every time a variable is accessed?

Question

Why is Square() being called again in the code below? I suppose values is being recalculated, but why? We got the values earlier, why is it being called again? Why does the compiler issue a warning "Possible multiple enumeration"?

static IEnumerable<int> Square(IEnumerable<int> a)
{
    foreach(var r in a)
    {
        Console.WriteLine($"From Square {r * r}");
        yield return r * r;
    }
}
class Wrap
{
    private static int init = 0;
    public int Value
    {
        get { return ++init; }
    }
}

static void Main(string[] args)
{
    var w = new Wrap();
    var wraps = new Wrap[3];
    for(int i=0; i<wraps.Length; i++)
    {
        wraps[i] = w;
    }

    var values = wraps.Select(x => x.Value);
    var results = Square(values);
    int sum = 0;
    int count = 0;
    foreach(var r in results)
    {
        count++;
        sum += r;
    }
    Console.WriteLine("Count {0}", count);
    Console.WriteLine("Sum {0}", sum);

    Console.WriteLine("Count {0}", results.Count());
    Console.WriteLine("Sum {0}", results.Sum());
}

Results:

From Square 1
From Square 4
From Square 9
Count 3
Sum 14
From Square 16
From Square 25
From Square 36
Count 3
From Square 49
From Square 64
From Square 81
Sum 194

Answer 1

Square(values) returns an IEnumerable, which is iterated over (called) each time it is accessed.

If you cast the result to a List(), the results are concreted into a List().

var results = Square(values).ToList();

Results with .ToList()

From Square 1
From Square 4
From Square 9
Count 3
Sum 14
Count 3
Sum 14