diff --git a/00_hello/hello.rb b/00_hello/hello.rb
new file mode 100644
index 000000000..61626e810
--- /dev/null
+++ b/00_hello/hello.rb
@@ -0,0 +1,7 @@
+def hello
+  "Hello!"
+end
+
+def greet(name)
+  "Hello, #{name}!"
+end
diff --git a/01_temperature/temperature.rb b/01_temperature/temperature.rb
new file mode 100644
index 000000000..d82b233da
--- /dev/null
+++ b/01_temperature/temperature.rb
@@ -0,0 +1,7 @@
+def ftoc(degrees)
+  (degrees - 32).to_f * 5.0 / 9.0
+end
+
+def ctof(d)
+  d * 9.0 / 5.0 + 32.0
+end
diff --git a/02_calculator/calculator.rb b/02_calculator/calculator.rb
new file mode 100644
index 000000000..09aa5c6c0
--- /dev/null
+++ b/02_calculator/calculator.rb
@@ -0,0 +1,27 @@
+def add(x, y)
+  x + y
+end
+
+def subtract(x, y)
+  x - y
+end
+
+def sum(nums)
+  nums.reduce(0){ |total, n| total + n }
+end
+
+def multiply(*nums)
+  nums.reduce(1){ |total, n| total * n }
+end
+
+def power(x, y)
+  x ** y
+end
+
+def factorial(x)
+  if x <= 1
+    1
+  else
+    x * factorial(x - 1)
+  end
+end
diff --git a/02_calculator/calculator_spec.rb b/02_calculator/calculator_spec.rb
index fef7e9d00..3ce77d87f 100644
--- a/02_calculator/calculator_spec.rb
+++ b/02_calculator/calculator_spec.rb
@@ -79,21 +79,41 @@
 
 describe "#multiply" do
 
-  it "multiplies two numbers"
+  it "multiplies two numbers" do
+    expect(multiply(3,3)).to eq(9)
+  end
+
+  it "multiplies several numbers" do
+    expect(multiply(3, 4, 5)).to eq(60)
+  end
 
-  it "multiplies several numbers"
-  
 end
 
 describe "#power" do
-  it "raises one number to the power of another number"
+  it "raises one number to the power of another number" do
+    expect(power(2, 3)).to eq(8)
+  end
 end
 
 # http://en.wikipedia.org/wiki/Factorial
 describe "#factorial" do
-  it "computes the factorial of 0"
-  it "computes the factorial of 1"
-  it "computes the factorial of 2"
-  it "computes the factorial of 5"
-  it "computes the factorial of 10"
+  it "computes the factorial of 0" do
+    expect(factorial(0)).to eq(1)
+  end
+
+  it "computes the factorial of 1" do
+    expect(factorial(1)).to eq(1)
+  end
+
+  it "computes the factorial of 2" do
+    expect(factorial(2)).to eq(2)
+  end
+
+  it "computes the factorial of 5" do
+    expect(factorial(5)).to eq(120)
+  end
+
+  it "computes the factorial of 10" do
+    expect(factorial(10)).to eq(3628800)
+  end
 end
diff --git a/03_simon_says/simon_says.rb b/03_simon_says/simon_says.rb
new file mode 100644
index 000000000..37a3d4353
--- /dev/null
+++ b/03_simon_says/simon_says.rb
@@ -0,0 +1,40 @@
+def echo(s)
+  s
+end
+
+def shout(s)
+  s.upcase
+end
+
+def repeat(s, times=2)
+  if times <= 1
+    s
+  else
+    s + " " + repeat(s, times - 1)
+  end
+end
+
+def start_of_word(s, length)
+  s[0...length]
+end
+
+def first_word(s)
+  s.split(" ")[0]
+end
+
+def titleize(s)
+  littlewords = ["the", "and", "over"]
+  split = s.split(" ")
+  capwords = split.each_with_index.map do |word, i|
+    if i == 0 && split[0] == word
+      word.capitalize
+    elsif littlewords.include?(word)
+      word
+    else
+      word.capitalize
+    end
+  end
+  capwords.join(" ")
+end
+
+titleize("the bridge over the river kwai")
diff --git a/04_pig_latin/pig_latin.rb b/04_pig_latin/pig_latin.rb
new file mode 100644
index 000000000..e6670ab96
--- /dev/null
+++ b/04_pig_latin/pig_latin.rb
@@ -0,0 +1,24 @@
+def translate(s)
+  vowels = "aeiou".split("")
+  split = s.split(" ")
+  split.reduce([]) do |acc, word|
+    # begins with vowel
+    if vowels.include?(word[0].downcase)
+      word = word + "ay"
+      # with 3 consonants
+    elsif word[0..2].split("").all?{ |char| not(vowels.include?(char)) } ||
+          (not(vowels.include?(word[0])) && word[1..2] == "qu")
+      word = word[3..-1] + word[0..2] + "ay"
+      # with 2 consonants
+    elsif word[0..1].split("").all?{ |char| not(vowels.include?(char)) } ||
+          word[0..1].downcase == "qu"
+      word = word[2..-1] + word[0..1] + "ay"
+      # with 1 consonant
+    else
+      word = word[1..-1] + word[0] + "ay"
+    end
+    acc.push(word)
+  end.join(" ")
+end
+
+translate("the quick brown fox")
diff --git a/05_silly_blocks/silly_blocks.rb b/05_silly_blocks/silly_blocks.rb
new file mode 100644
index 000000000..b9650b5b4
--- /dev/null
+++ b/05_silly_blocks/silly_blocks.rb
@@ -0,0 +1,13 @@
+def reverser
+  yield.split(" ").map(&:reverse).join(" ")
+end
+
+def adder(n=1)
+  yield + n
+end
+
+def repeater(times=1)
+  times.times do
+    yield
+  end
+end
diff --git a/06_performance_monitor/performance_monitor.rb b/06_performance_monitor/performance_monitor.rb
new file mode 100644
index 000000000..4f2b0faa8
--- /dev/null
+++ b/06_performance_monitor/performance_monitor.rb
@@ -0,0 +1,11 @@
+require "time"
+
+def measure(times=1)
+  acc = []
+  times.times do
+    start = Time.now
+    yield
+    acc.push(Time.now - start)
+  end
+  acc.reduce(&:+).to_f / times.to_f
+end
diff --git a/07_hello_friend/friend.rb b/07_hello_friend/friend.rb
new file mode 100644
index 000000000..cf9f4bbdb
--- /dev/null
+++ b/07_hello_friend/friend.rb
@@ -0,0 +1,9 @@
+class Friend
+  def greeting(name=nil)
+    if name
+      "Hello, #{name}!"
+    else
+      "Hello!"
+    end
+  end
+end
diff --git a/08_book_titles/book.rb b/08_book_titles/book.rb
new file mode 100644
index 000000000..363af456b
--- /dev/null
+++ b/08_book_titles/book.rb
@@ -0,0 +1,24 @@
+class Book
+
+  attr_reader :title, :app_state
+
+  def initialize
+    @app_state = {
+      exceptions: "the a an and in of".split(" ")
+    }
+    @title = nil
+  end
+
+  def title=(s)
+    @title = s.split(" ").each_with_index.map do |word, i|
+      if i == 0
+        word.capitalize
+      elsif app_state[:exceptions].include?(word)
+        word
+      else
+        word.capitalize
+      end
+    end.join(" ")
+  end
+
+end
diff --git a/09_timer/timer.rb b/09_timer/timer.rb
new file mode 100644
index 000000000..ae879e79e
--- /dev/null
+++ b/09_timer/timer.rb
@@ -0,0 +1,22 @@
+class Timer
+  attr_accessor :seconds
+
+  def initialize
+    @seconds = 0
+  end
+
+  def pre_zero(int)
+    if int.to_s.length == 1
+      "0#{int}"
+    else
+      int.to_s
+    end
+  end
+
+  def time_string
+    seconds_left = seconds % 60
+    minutes_left = (seconds/60) % 60
+    hour = (seconds/60) / 60
+    "#{pre_zero(hour)}:#{pre_zero(minutes_left)}:#{pre_zero(seconds_left)}"
+  end
+end
diff --git a/10_temperature_object/temperature.rb b/10_temperature_object/temperature.rb
new file mode 100644
index 000000000..efd79451d
--- /dev/null
+++ b/10_temperature_object/temperature.rb
@@ -0,0 +1,89 @@
+def ftoc(degrees)
+  (degrees - 32).to_f * 5.0 / 9.0
+end
+
+def ctof(d)
+  d * 9.0 / 5.0 + 32.0
+end
+
+# ----------------------------------------------------------------------
+# Temperature
+# ----------------------------------------------------------------------
+
+class Temperature
+
+  attr_accessor :degrees
+
+  def initialize(options = {})
+    @degrees = options
+  end
+
+  def self.from_celsius(d)
+    Temperature.new({c: d})
+  end
+
+  def self.from_fahrenheit(d)
+    Temperature.new({f: d})
+  end
+
+  def in_fahrenheit
+    if degrees[:f]
+      degrees[:f]
+    else
+      ctof(degrees[:c])
+    end
+  end
+
+  def in_celsius
+    if degrees[:f]
+      ftoc(degrees[:f])
+    else
+      degrees[:c]
+    end
+  end
+
+end
+
+# ----------------------------------------------------------------------
+# Celsius
+# ----------------------------------------------------------------------
+
+class Celsius < Temperature
+
+  attr_accessor :degree
+
+  def initialize(d)
+    @degree = d
+  end
+
+  def in_celsius
+    degree
+  end
+
+  def in_fahrenheit
+    ctof(degree)
+  end
+
+end
+
+# ----------------------------------------------------------------------
+# Fahrenheit
+# ----------------------------------------------------------------------
+
+class Fahrenheit < Temperature
+
+  attr_accessor :degree
+
+  def initialize(d)
+    @degree = d
+  end
+
+  def in_fahrenheit
+    degree
+  end
+
+  def in_celsius
+    ftoc(degree)
+  end
+
+end
diff --git a/11_dictionary/dictionary.rb b/11_dictionary/dictionary.rb
new file mode 100644
index 000000000..b0453bb93
--- /dev/null
+++ b/11_dictionary/dictionary.rb
@@ -0,0 +1,41 @@
+class Dictionary
+
+  attr_accessor :entries
+
+  def initialize
+    @entries = {}
+  end
+
+  def add(keyvals)
+    if keyvals.is_a?(Hash)
+      @entries = entries.merge(keyvals)
+    else
+      @entries = entries.merge({keyvals => nil})
+    end
+  end
+
+  def keywords
+    entries.keys.sort
+  end
+
+  def include?(k)
+    entries.include?(k)
+  end
+
+  def find(s)
+    entries.select{ |k, v| k.include?(s) }
+  end
+
+  def printable
+    entries.sort.reduce("") do |acc, kv|
+      k, v = kv
+      s = %Q{[#{k}] "#{v}"}
+      if acc.empty?
+        s
+      else
+        acc + "\n" + s
+      end
+    end
+  end
+
+end
diff --git a/12_rpn_calculator/rpn_calculator.rb b/12_rpn_calculator/rpn_calculator.rb
new file mode 100644
index 000000000..c97ea47b8
--- /dev/null
+++ b/12_rpn_calculator/rpn_calculator.rb
@@ -0,0 +1,91 @@
+class RPNCalculator
+
+  attr_accessor :stack, :value
+
+  def initialize
+    @stack = []
+    @value = 0
+  end
+
+  def tokens(s)
+    operators = "* + - /".split(" ")
+    s.split(" ").map do |char|
+      if operators.include?(char)
+        char.to_sym
+      else
+        char.to_i
+      end
+    end
+  end
+
+  def map_operators(op)
+    {
+      :* => method(:times),
+      :/ => method(:divide),
+      :- => method(:minus),
+      :+ => method(:plus)
+    }[op]
+  end
+
+  def evaluate(s)
+    operators = [:*, :/, :+, :-]
+    stack_ = tokens(s)
+    stack_.each do |i|
+      if i.is_a?(Integer)
+        push(i)
+      elsif operators.include?(i)
+        map_operators(i).call
+      end
+    end
+    value
+  end
+
+  def empty_stack_error
+    raise "calculator is empty"
+  end
+
+  def push(n)
+    stack.push(n)
+  end
+
+  def plus
+    if stack.empty?
+      empty_stack_error
+    end
+    x, y = stack.pop, stack.pop
+    result = x + y
+    @value = result
+    push(result)
+  end
+
+  def minus
+    if stack.empty?
+      empty_stack_error
+    end
+    x, y = stack.pop, stack.pop
+    result = y - x
+    @value = result
+    push(result)
+  end
+
+  def divide
+    if stack.empty?
+      empty_stack_error
+    end
+    x, y = stack.pop, stack.pop
+    result = y.to_f / x.to_f
+    @value = result
+    push(result)
+  end
+
+  def times
+    if stack.empty?
+      empty_stack_error
+    end
+    x, y = stack.pop, stack.pop
+    result = x * y
+    @value = result
+    push(result)
+  end
+
+end
diff --git a/14_array_extensions/array_extensions.rb b/14_array_extensions/array_extensions.rb
new file mode 100644
index 000000000..8d237a6de
--- /dev/null
+++ b/14_array_extensions/array_extensions.rb
@@ -0,0 +1,17 @@
+class Array
+
+  def square
+    if self.empty?
+      []
+    else
+      self.map { |i| i*i  }
+    end
+  end
+
+  def square!
+    self.each_with_index do |item, i|
+      self[i] = item*item
+    end
+  end
+
+end
diff --git a/15_in_words/in_words.rb b/15_in_words/in_words.rb
new file mode 100644
index 000000000..086fdbf42
--- /dev/null
+++ b/15_in_words/in_words.rb
@@ -0,0 +1,151 @@
+class Integer
+
+  def one_to_ten
+    case self
+    when 0
+      "zero"
+    when 1
+      "one"
+    when 2
+      "two"
+    when 3
+      "three"
+    when 4
+      "four"
+    when 5
+      "five"
+    when 6
+      "six"
+    when 7
+      "seven"
+    when 8
+      "eight"
+    when 9
+      "nine"
+    end
+  end
+
+  def ten_to_twelve
+    case self
+    when 10
+      "ten"
+    when 11
+      "eleven"
+    when 12
+      "twelve"
+    end
+  end
+
+  def teens
+    case self
+    when 13
+      "thirteen"
+    when 14
+      "fourteen"
+    when 15
+      "fifteen"
+    when 16
+      "sixteen"
+    when 17
+      "seventeen"
+    when 18
+      "eighteen"
+    when 19
+      "nineteen"
+    end
+  end
+
+  def tens
+    case self
+    when 20
+      "twenty"
+    when 30
+      "thirty"
+    when 40
+      "forty"
+    when 50
+      "fifty"
+    when 60
+      "sixty"
+    when 70
+      "seventy"
+    when 80
+      "eighty"
+    when 90
+      "ninety"
+    end
+  end
+
+  def hundreds
+    n, rem = self / 100, self % 100
+    r = "#{n.in_words} hundred"
+    r += " " + rem.in_words if rem > 0
+    r
+  end
+
+  def decimalize
+    self.to_s.split("").reverse.each_with_index.map do |s, i|
+      s.to_i * (10**i)
+    end.reverse
+  end
+
+  def others_less_than_hundred
+    self.decimalize.map(&:in_words).join(" ")
+  end
+
+  def twenty_below_hundred
+    tens || others_less_than_hundred
+  end
+
+  def thousands
+    n, rem = self / 1000, self % 1000
+    r = "#{n.in_words} thousand"
+    r += " " + rem.in_words if rem > 0
+    r
+  end
+
+  def millions
+    n, rem = self / 1_000_000, self % 1_000_000
+    r = "#{n.in_words} million"
+    r += " " + rem.in_words if rem > 0
+    r
+  end
+
+  def billions
+    n, rem = self / 1_000_000_000, self % 1_000_000_000
+    r = "#{n.in_words} billion"
+    r += " " + rem.in_words if rem > 0
+    r
+  end
+
+  def trillions
+    n, rem = self / 1_000_000_000_000, self % 1_000_000_000_000
+    r = "#{n.in_words} trillion"
+    r += " " + rem.in_words if rem > 0
+    r
+  end
+
+  def in_words
+    case self
+    when 0..9
+      one_to_ten
+    when 10..12
+      ten_to_twelve
+    when 13..19
+      teens
+    when 20..99
+      twenty_below_hundred
+    when 100..999
+      hundreds
+    when 1000..999_999
+      thousands
+    when 1_000_000..999_999_999
+      millions
+    when 1_234_567_890..999_999_999_999
+      billions
+    else
+      trillions
+    end
+  end
+
+end
diff --git a/15_in_words/in_words_spec.rb b/15_in_words/in_words_spec.rb
index 1f5768575..09ca9d882 100644
--- a/15_in_words/in_words_spec.rb
+++ b/15_in_words/in_words_spec.rb
@@ -20,7 +20,7 @@
 
 require "in_words"
 
-describe Fixnum do
+describe Integer do
 
   it "reads 0 to 9" do
     expect(0.in_words).to eq('zero')