Silly list based noun finder

noun.rb

class String
  @@nouns = []

  def is_noun?
    String.nouns.include? self.downcase
  end

  def find_nouns
    self.split(/\s/).collect { |w| w if w.is_noun? }.compact
  end

  def self.nouns
    if @@nouns.empty?
      File.open('nouns_list.txt','r') do |f|
        f.each_line do |line|
          @@nouns.push line.strip.downcase
        end
      end
    end
    @@nouns
  end
end

input = IO.read('input.txt')

puts input.to_s.find_nouns