diff --git a/interfaces/04-interfaces/puzzle.go b/interfaces/04-interfaces/puzzle.go
index 5ea12b9..e839e54 100644
--- a/interfaces/04-interfaces/puzzle.go
+++ b/interfaces/04-interfaces/puzzle.go
@@ -14,6 +14,8 @@ type puzzle struct {
 	price money
 }
 
+// if you remove this method,
+// you can no longer add it to the `store` in `main()`.
 func (p puzzle) print() {
 	fmt.Printf("%-15s: %s\n", p.title, p.price.string())
 }
diff --git a/interfaces/05-assertion/book.go b/interfaces/05-assertion/book.go
index a55520f..9b0ebaa 100644
--- a/interfaces/05-assertion/book.go
+++ b/interfaces/05-assertion/book.go
@@ -10,9 +10,8 @@ package main
 import "fmt"
 
 type book struct {
-	title    string
-	price    money
-	readTime int
+	title string
+	price money
 }
 
 func (b book) print() {
diff --git a/interfaces/05-assertion/game.go b/interfaces/05-assertion/game.go
index 2705bee..b51bf39 100644
--- a/interfaces/05-assertion/game.go
+++ b/interfaces/05-assertion/game.go
@@ -10,9 +10,8 @@ package main
 import "fmt"
 
 type game struct {
-	title    string
-	price    money
-	playTime int
+	title string
+	price money
 }
 
 func (g *game) print() {
diff --git a/interfaces/05-assertion/list.go b/interfaces/05-assertion/list.go
index 8a3cbf7..ad73fe9 100644
--- a/interfaces/05-assertion/list.go
+++ b/interfaces/05-assertion/list.go
@@ -11,6 +11,9 @@ import "fmt"
 
 type printer interface {
 	print()
+
+	// use type assertion when you cannot change the interface.
+	// discount(ratio float64)
 }
 
 type list []printer
@@ -26,59 +29,50 @@ func (l list) print() {
 	}
 }
 
+// type assertion can extract the wrapped value.
+// or: it can put the value into another interface.
 func (l list) discount(ratio float64) {
+	// you can declare an interface in a function/method as well.
+	// interface is just a type.
 	type discounter interface {
 		discount(float64)
 	}
 
 	for _, it := range l {
+		// you can assert to an interface.
+		// and extract another interface.
 		if it, ok := it.(discounter); ok {
 			it.discount(ratio)
 		}
 	}
 }
 
-//
-// type assertion can pull out the real value behind an interface value.
-//
-// it can also check whether the value convertable to a given type.
-//
+// ----
+
 // func (l list) discount(ratio float64) {
 // 	for _, it := range l {
-// 		// remember: interface is a type
-// 		if it, ok := it.(interface{ discount(float64) }); ok {
-// 			it.discount(ratio)
+// 		// you can inline-assert interfaces
+// 		// interface is just a type.
+// 		g, ok := it.(interface{ discount(float64) })
+// 		if !ok {
+// 			continue
 // 		}
+
+// 		g.discount(ratio)
 // 	}
 // }
 
-//
-// type switch can pull out the real value behind an interface value.
-//
-// func (l list) discount(ratio float64) {
-// 	for _, it := range l {
-// 		switch it := it.(type) {
-// 		case *game:
-// 			it.discount(ratio)
-// 		case *puzzle:
-// 			it.discount(ratio)
-// 		}
-// 	}
-// }
+// ----
 
-//
-// type switch can find the type behind an interface value.
-//
+// // type assertion can pull out the real value behind an interface value.
+// // it can also check whether the value convertable to a given type.
 // func (l list) discount(ratio float64) {
 // 	for _, it := range l {
-// 		switch it.(type) {
-// 		case *game:
-// 			fmt.Print("it's a *game!   --> ")
-// 		case *puzzle:
-// 			fmt.Print("it's a *puzzle! --> ")
-// 		default:
-// 			fmt.Print("neither         --> ")
+// 		g, ok := it.(*game)
+// 		if !ok {
+// 			continue
 // 		}
-// 		it.print()
+
+// 		g.discount(ratio)
 // 	}
 // }
diff --git a/interfaces/05-assertion/main.go b/interfaces/05-assertion/main.go
index bde7e9c..9eeafad 100644
--- a/interfaces/05-assertion/main.go
+++ b/interfaces/05-assertion/main.go
@@ -13,16 +13,19 @@ func main() {
 		minecraft = game{title: "minecraft", price: 20}
 		tetris    = game{title: "tetris", price: 5}
 		rubik     = puzzle{title: "rubik's cube", price: 5}
+		yoda      = toy{title: "yoda", price: 150}
 	)
 
 	var store list
-	store = append(store, &minecraft, &tetris, mobydick, &rubik)
+	store = append(store, &minecraft, &tetris, mobydick, rubik, &yoda)
 
+	// #2
 	store.discount(.5)
 	store.print()
 
-	// minecraft.discount(.5)
-	// minecraft.print()
+	// #1
+	// var p printer
+	// p = &tetris
+	// tetris.discount(.5)
+	// p.print()
 }
-
-// store.discount()
diff --git a/interfaces/05-assertion/puzzle.go b/interfaces/05-assertion/puzzle.go
index de54585..5ea12b9 100644
--- a/interfaces/05-assertion/puzzle.go
+++ b/interfaces/05-assertion/puzzle.go
@@ -14,10 +14,6 @@ type puzzle struct {
 	price money
 }
 
-func (p *puzzle) print() {
+func (p puzzle) print() {
 	fmt.Printf("%-15s: %s\n", p.title, p.price.string())
 }
-
-func (p *puzzle) discount(ratio float64) {
-	p.price *= money(1 - ratio)
-}
diff --git a/interfaces/05-assertion/toy.go b/interfaces/05-assertion/toy.go
new file mode 100644
index 0000000..9467e69
--- /dev/null
+++ b/interfaces/05-assertion/toy.go
@@ -0,0 +1,23 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type toy struct {
+	title string
+	price money
+}
+
+func (t *toy) print() {
+	fmt.Printf("%-15s: %s\n", t.title, t.price.string())
+}
+
+func (t *toy) discount(ratio float64) {
+	t.price *= money(1 - ratio)
+}
diff --git a/interfaces/06-type-switch/book.go b/interfaces/06-type-switch/book.go
new file mode 100644
index 0000000..9b0ebaa
--- /dev/null
+++ b/interfaces/06-type-switch/book.go
@@ -0,0 +1,19 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type book struct {
+	title string
+	price money
+}
+
+func (b book) print() {
+	fmt.Printf("%-15s: %s\n", b.title, b.price.string())
+}
diff --git a/interfaces/06-type-switch/game.go b/interfaces/06-type-switch/game.go
new file mode 100644
index 0000000..b51bf39
--- /dev/null
+++ b/interfaces/06-type-switch/game.go
@@ -0,0 +1,23 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type game struct {
+	title string
+	price money
+}
+
+func (g *game) print() {
+	fmt.Printf("%-15s: %s\n", g.title, g.price.string())
+}
+
+func (g *game) discount(ratio float64) {
+	g.price *= money(1 - ratio)
+}
diff --git a/interfaces/06-type-switch/list.go b/interfaces/06-type-switch/list.go
new file mode 100644
index 0000000..5743b60
--- /dev/null
+++ b/interfaces/06-type-switch/list.go
@@ -0,0 +1,96 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type printer interface {
+	print()
+	// discount(ratio float64)
+}
+
+type list []printer
+
+func (l list) print() {
+	if len(l) == 0 {
+		fmt.Println("Sorry. Our store is closed. We're waiting for the delivery 🚚.")
+		return
+	}
+
+	for _, it := range l {
+		it.print()
+	}
+}
+
+//
+// type switch can find the type behind an interface value.
+//
+func (l list) discount(ratio float64) {
+	type discounter interface {
+		discount(float64)
+	}
+
+	for _, it := range l {
+		if it, ok := it.(discounter); ok {
+			it.discount(ratio)
+		}
+	}
+}
+
+// ----
+
+//
+// type assertion can pull out the real value behind an interface value.
+//
+// it can also check whether the value convertable to a given type.
+//
+// func (l list) discount(ratio float64) {
+// 	for _, it := range l {
+// 		g, ok := it.(interface{ discount(float64) })
+// 		if !ok {
+// 			continue
+// 		}
+
+// 		g.discount(ratio)
+// 	}
+// }
+
+// ----
+
+//
+// type switch can pull out the real value behind an interface value.
+//
+// func (l list) discount(ratio float64) {
+// 	for _, it := range l {
+// 		switch it := it.(type) {
+// 		case *game:
+// 			it.discount(ratio)
+// 		case *puzzle:
+// 			it.discount(ratio)
+// 		}
+// 	}
+// }
+
+// ----
+
+//
+// type switch can find the type behind an interface value.
+//
+// func (l list) discount(ratio float64) {
+// 	for _, it := range l {
+// 		switch it.(type) {
+// 		case *game:
+// 			fmt.Print("it's a *game!   --> ")
+// 		case *puzzle:
+// 			fmt.Print("it's a *puzzle! --> ")
+// 		default:
+// 			fmt.Print("neither         --> ")
+// 		}
+// 		it.print()
+// 	}
+// }
diff --git a/interfaces/06-type-switch/main.go b/interfaces/06-type-switch/main.go
new file mode 100644
index 0000000..9eeafad
--- /dev/null
+++ b/interfaces/06-type-switch/main.go
@@ -0,0 +1,31 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+func main() {
+	var (
+		mobydick  = book{title: "moby dick", price: 10}
+		minecraft = game{title: "minecraft", price: 20}
+		tetris    = game{title: "tetris", price: 5}
+		rubik     = puzzle{title: "rubik's cube", price: 5}
+		yoda      = toy{title: "yoda", price: 150}
+	)
+
+	var store list
+	store = append(store, &minecraft, &tetris, mobydick, rubik, &yoda)
+
+	// #2
+	store.discount(.5)
+	store.print()
+
+	// #1
+	// var p printer
+	// p = &tetris
+	// tetris.discount(.5)
+	// p.print()
+}
diff --git a/interfaces/06-composition/money.go b/interfaces/06-type-switch/money.go
similarity index 100%
rename from interfaces/06-composition/money.go
rename to interfaces/06-type-switch/money.go
diff --git a/interfaces/06-type-switch/puzzle.go b/interfaces/06-type-switch/puzzle.go
new file mode 100644
index 0000000..5ea12b9
--- /dev/null
+++ b/interfaces/06-type-switch/puzzle.go
@@ -0,0 +1,19 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type puzzle struct {
+	title string
+	price money
+}
+
+func (p puzzle) print() {
+	fmt.Printf("%-15s: %s\n", p.title, p.price.string())
+}
diff --git a/interfaces/06-type-switch/toy.go b/interfaces/06-type-switch/toy.go
new file mode 100644
index 0000000..9467e69
--- /dev/null
+++ b/interfaces/06-type-switch/toy.go
@@ -0,0 +1,23 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type toy struct {
+	title string
+	price money
+}
+
+func (t *toy) print() {
+	fmt.Printf("%-15s: %s\n", t.title, t.price.string())
+}
+
+func (t *toy) discount(ratio float64) {
+	t.price *= money(1 - ratio)
+}
diff --git a/interfaces/06-composition/book.go b/interfaces/07-composition/book.go
similarity index 100%
rename from interfaces/06-composition/book.go
rename to interfaces/07-composition/book.go
diff --git a/interfaces/06-composition/discount.go b/interfaces/07-composition/discount.go
similarity index 100%
rename from interfaces/06-composition/discount.go
rename to interfaces/07-composition/discount.go
diff --git a/interfaces/06-composition/game.go b/interfaces/07-composition/game.go
similarity index 100%
rename from interfaces/06-composition/game.go
rename to interfaces/07-composition/game.go
diff --git a/interfaces/06-composition/list.go b/interfaces/07-composition/list.go
similarity index 100%
rename from interfaces/06-composition/list.go
rename to interfaces/07-composition/list.go
diff --git a/interfaces/06-composition/main.go b/interfaces/07-composition/main.go
similarity index 100%
rename from interfaces/06-composition/main.go
rename to interfaces/07-composition/main.go
diff --git a/interfaces/07-composition/money.go b/interfaces/07-composition/money.go
new file mode 100644
index 0000000..86f7d63
--- /dev/null
+++ b/interfaces/07-composition/money.go
@@ -0,0 +1,16 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type money float64
+
+func (m money) string() string {
+	return fmt.Sprintf("$%.2f", m)
+}
diff --git a/interfaces/06-composition/puzzle.go b/interfaces/07-composition/puzzle.go
similarity index 100%
rename from interfaces/06-composition/puzzle.go
rename to interfaces/07-composition/puzzle.go
diff --git a/interfaces/06-composition/time.go b/interfaces/07-composition/time.go
similarity index 100%
rename from interfaces/06-composition/time.go
rename to interfaces/07-composition/time.go
diff --git a/interfaces/06-composition/types.go b/interfaces/07-composition/types.go
similarity index 100%
rename from interfaces/06-composition/types.go
rename to interfaces/07-composition/types.go