How are values actually fetched from an Enumerator object in Ruby?

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I'm interested in how values are fetched from an Enumerator object. In the following piece of code, I was expecting the first enum.next call to raise an exception as all values had already been received from enum after the call to enum.to_a.

enum = Enumerator.new do |yielder|
  yielder.yield 1
  yielder.yield 2
  yielder.yield 3
end

p enum.to_a # => [1, 2, 3]

puts enum.next # Expected StopIteration here
puts enum.next
puts enum.next
puts enum.next # => StopIteration exception raised

What's the difference between calling next vs an iterator method like to_a on an instance of Enumerator?

enumerator ruby
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