1_open.rb

class Fixnum
  def seconds
    self
  end

  def minutes
    self * 60
  end

  def hours
    minutes * 60
  end

  def days
    hours * 24
  end

  def ago
    Time.now - self
  end

  def from_now
    Time.now + self
  end
end

puts Time.now        # => Thu Dec 23 09:41:04 +0100 2010
puts 3.days.from_now # => Sun Dec 26 09:41:06 +0100 2010

2_conditional.rb

class Condition < Proc
  def ===(object)
    call(object)
  end
end

even = Condition.new { |x| x % 2 == 0 }
odd  = Condition.new { |x| x % 2 != 0 }

case 3
when even
  puts "Even!"
when odd
  puts "Odd!"
end

3_builder.rb

require 'rubygems'
require 'builder'

builder = Builder::XmlMarkup.new

builder.person do |b|
  b.name("Jim")
  b.phone("555-1234")
end

# Prints:
# <person>
#   <name>Jim</name>
#   <phone>555-1234</phone>
# </person>