no such file to load -- nokogiri (LoadError)

  require 'nokogiri'
  require 'open-uri'

  # Get a Nokogiri::HTML:Document for the page we’re interested in...

  doc = Nokogiri::HTML(open('http://www.google.com/search?q=tenderlove'))

  # Do funky things with it using Nokogiri::XML::Node methods...

  ####
  # Search for nodes by css
  doc.css('h3.r a.l').each do |link|
    puts link.content
  end

  ####
  # Search for nodes by xpath
  doc.xpath('//h3/a[@class="l"]').each do |link|
    puts link.content
  end

  ####
  # Or mix and match.
  doc.search('h3.r a.l', '//h3/a[@class="l"]').each do |link|
    puts link.content
  end

执行以上代码报如下错误：
no such file to load -- nokogiri (LoadError)

第一行加入下面代码即可：

require 'rubygems'