diff --git a/README.md b/README.md
index 398ec0a..8cafe13 100644
--- a/README.md
+++ b/README.md
@@ -19,3 +19,4 @@
   * [第09章 歩幅の調整](https://github.com/at-grandpa/study-tdd/pull/13)
   * [第10章 テストに聞いてみる](https://github.com/at-grandpa/study-tdd/pull/14)
   * [第11章 不要になったら消す](https://github.com/at-grandpa/study-tdd/pull/15)
+  * [第12章 設計とメタファー](https://github.com/at-grandpa/study-tdd/pull/16)
diff --git a/part01/spec/money_package_spec.cr b/part01/spec/money_package_spec.cr
index 7ee6937..b390cb8 100644
--- a/part01/spec/money_package_spec.cr
+++ b/part01/spec/money_package_spec.cr
@@ -23,4 +23,13 @@ describe MoneyPackage do
       Money.franc(1).currency.should eq "CHF"
     end
   end
+  describe "testSimpleAddition()" do
+    it "足し算を計算できること" do
+      five = Money.dollar(5)
+      sum : Expression = five.plus(five)
+      bank = Bank.new
+      reduced = bank.reduce(sum, "USD")
+      reduced.should eq Money.dollar(10)
+    end
+  end
 end
diff --git a/part01/src/money_package/bank.cr b/part01/src/money_package/bank.cr
new file mode 100644
index 0000000..8586327
--- /dev/null
+++ b/part01/src/money_package/bank.cr
@@ -0,0 +1,7 @@
+module MoneyPackage
+  class Bank
+    def reduce(source : Expression, to : String)
+      Money.dollar(10)
+    end
+  end
+end
diff --git a/part01/src/money_package/expression.cr b/part01/src/money_package/expression.cr
new file mode 100644
index 0000000..28b9362
--- /dev/null
+++ b/part01/src/money_package/expression.cr
@@ -0,0 +1,4 @@
+module MoneyPackage
+  module Expression
+  end
+end
diff --git a/part01/src/money_package/money.cr b/part01/src/money_package/money.cr
index e413124..f0e12d1 100644
--- a/part01/src/money_package/money.cr
+++ b/part01/src/money_package/money.cr
@@ -1,5 +1,9 @@
+require "./expression"
+
 module MoneyPackage
   class Money
+    include Expression
+
     def initialize(@amount : Int32, @currency : String)
     end
 
@@ -15,6 +19,10 @@ module MoneyPackage
       Money.new(@amount * multiplier, @currency)
     end
 
+    def plus(addend : self) : Expression
+      Money.new(@amount + addend.@amount, @currency)
+    end
+
     def self.dollar(amount : Int32) : self
       Money.new(amount, "USD")
     end