diff --git a/interfaces/01-methods/main.go b/interfaces/01-methods/main.go
index dc3a19b..0b751ab 100644
--- a/interfaces/01-methods/main.go
+++ b/interfaces/01-methods/main.go
@@ -7,8 +7,6 @@
 
 package main
 
-import "fmt"
-
 func main() {
 	mobydick := book{
 		title: "moby dick",
@@ -25,12 +23,6 @@ func main() {
 		price: 5,
 	}
 
-	// #4: method expressions
-	// methods are just functions that receive a value of a type.
-	game.print(minecraft) // sends `minecraft` value to `game.print`
-	game.print(tetris)    // sends `tetris` value to `game.print`
-	fmt.Println()
-
 	// #3
 	mobydick.print()  // sends `mobydick` value to `book.print`
 	minecraft.print() // sends `minecraft` value to `game.print`
diff --git a/interfaces/02-receivers/book.go b/interfaces/02-receivers/book.go
index 7b925aa..2d01091 100644
--- a/interfaces/02-receivers/book.go
+++ b/interfaces/02-receivers/book.go
@@ -15,6 +15,5 @@ type book struct {
 }
 
 func (b book) print() {
-	// b is a copy of the original `book` value here.
 	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
 }
diff --git a/interfaces/02-receivers/game.go b/interfaces/02-receivers/game.go
index dcaf1cf..0dc2b3b 100644
--- a/interfaces/02-receivers/game.go
+++ b/interfaces/02-receivers/game.go
@@ -14,6 +14,9 @@ type game struct {
 	price float64
 }
 
+// be consistent:
+// if another method in the same type has a pointer receiver,
+// use pointer receivers on the other methods as well.
 func (g *game) print() {
 	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
 }
@@ -27,8 +30,7 @@ func (g *game) discount(ratio float64) {
 
 // PREVIOUS CODE:
 
-// ----------------------------------------------------------------------------
 // + `g` is a copy: `discount` cannot change the original `g`.
-// func (g game) discount(ratio float64) {
+// func (g *game) discount(ratio float64) {
 // 	g.price *= (1 - ratio)
 // }
diff --git a/interfaces/02-receivers/huge.go b/interfaces/02-receivers/huge.go
new file mode 100644
index 0000000..340a2c1
--- /dev/null
+++ b/interfaces/02-receivers/huge.go
@@ -0,0 +1,29 @@
+// 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 huge struct {
+	// about ~200mb
+	games [10000000]game
+}
+
+// only copies a single pointer.
+func (h *huge) addr() {
+	fmt.Printf("%p\n", h)
+}
+
+// each time it is called,
+// the original value will be copied.
+// calling addr() x 10 times = ~2 GB.
+// func (h huge) addr() {
+// 	fmt.Printf("%p\n", &h)
+// }
diff --git a/interfaces/02-receivers/main.go b/interfaces/02-receivers/main.go
index deba14e..e4ecb30 100644
--- a/interfaces/02-receivers/main.go
+++ b/interfaces/02-receivers/main.go
@@ -9,19 +9,22 @@ package main
 
 func main() {
 	var (
-		// mobydick  = book{title: "moby dick", price: 10}
+		mobydick  = book{title: "moby dick", price: 10}
 		minecraft = game{title: "minecraft", price: 20}
 		tetris    = game{title: "tetris", price: 5}
 	)
 
-	var items []game
-	items = append(items, minecraft, tetris)
+	// sends a pointer of minecraft to the discount method
+	// same as: (&minecraft).discount(.1)
+	minecraft.discount(.1)
 
-	// you can attach methods to a compatible type on the fly:
-	my := list([]game{minecraft, tetris})
+	mobydick.print()
+	minecraft.print()
+	tetris.print()
 
-	// you can call methods even on a nil value
-	// my = nil
-
-	my.print() // or: list.print(items)
+	// creates a variable that occupies 200mb on memory
+	var h huge
+	for i := 0; i < 10; i++ {
+		h.addr()
+	}
 }
diff --git a/interfaces/03-nonstructs/book.go b/interfaces/03-nonstructs/book.go
new file mode 100644
index 0000000..7b925aa
--- /dev/null
+++ b/interfaces/03-nonstructs/book.go
@@ -0,0 +1,20 @@
+// 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 float64
+}
+
+func (b book) print() {
+	// b is a copy of the original `book` value here.
+	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
+}
diff --git a/interfaces/03-nonstructs/game.go b/interfaces/03-nonstructs/game.go
new file mode 100644
index 0000000..dcaf1cf
--- /dev/null
+++ b/interfaces/03-nonstructs/game.go
@@ -0,0 +1,34 @@
+// 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 float64
+}
+
+func (g *game) print() {
+	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+}
+
+// + discount gets a copy of `*game`.
+// + discount can update the original `game` through the game pointer.
+// + it's better to use the same receiver type: `*game` for all methods.
+func (g *game) discount(ratio float64) {
+	g.price *= (1 - ratio)
+}
+
+// PREVIOUS CODE:
+
+// ----------------------------------------------------------------------------
+// + `g` is a copy: `discount` cannot change the original `g`.
+// func (g game) discount(ratio float64) {
+// 	g.price *= (1 - ratio)
+// }
diff --git a/interfaces/02-receivers/list.go b/interfaces/03-nonstructs/list.go
similarity index 100%
rename from interfaces/02-receivers/list.go
rename to interfaces/03-nonstructs/list.go
diff --git a/interfaces/03-nonstructs/main.go b/interfaces/03-nonstructs/main.go
new file mode 100644
index 0000000..deba14e
--- /dev/null
+++ b/interfaces/03-nonstructs/main.go
@@ -0,0 +1,27 @@
+// 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}
+	)
+
+	var items []game
+	items = append(items, minecraft, tetris)
+
+	// you can attach methods to a compatible type on the fly:
+	my := list([]game{minecraft, tetris})
+
+	// you can call methods even on a nil value
+	// my = nil
+
+	my.print() // or: list.print(items)
+}
diff --git a/interfaces/03-interfaces/book.go b/interfaces/04-interfaces/book.go
similarity index 100%
rename from interfaces/03-interfaces/book.go
rename to interfaces/04-interfaces/book.go
diff --git a/interfaces/03-interfaces/game.go b/interfaces/04-interfaces/game.go
similarity index 100%
rename from interfaces/03-interfaces/game.go
rename to interfaces/04-interfaces/game.go
diff --git a/interfaces/03-interfaces/list.go b/interfaces/04-interfaces/list.go
similarity index 100%
rename from interfaces/03-interfaces/list.go
rename to interfaces/04-interfaces/list.go
diff --git a/interfaces/03-interfaces/main.go b/interfaces/04-interfaces/main.go
similarity index 100%
rename from interfaces/03-interfaces/main.go
rename to interfaces/04-interfaces/main.go
diff --git a/interfaces/03-interfaces/puzzle.go b/interfaces/04-interfaces/puzzle.go
similarity index 100%
rename from interfaces/03-interfaces/puzzle.go
rename to interfaces/04-interfaces/puzzle.go
diff --git a/interfaces/04-assertion/book.go b/interfaces/05-assertion/book.go
similarity index 100%
rename from interfaces/04-assertion/book.go
rename to interfaces/05-assertion/book.go
diff --git a/interfaces/04-assertion/game.go b/interfaces/05-assertion/game.go
similarity index 100%
rename from interfaces/04-assertion/game.go
rename to interfaces/05-assertion/game.go
diff --git a/interfaces/04-assertion/list.go b/interfaces/05-assertion/list.go
similarity index 100%
rename from interfaces/04-assertion/list.go
rename to interfaces/05-assertion/list.go
diff --git a/interfaces/04-assertion/main.go b/interfaces/05-assertion/main.go
similarity index 100%
rename from interfaces/04-assertion/main.go
rename to interfaces/05-assertion/main.go
diff --git a/interfaces/04-assertion/puzzle.go b/interfaces/05-assertion/puzzle.go
similarity index 100%
rename from interfaces/04-assertion/puzzle.go
rename to interfaces/05-assertion/puzzle.go
diff --git a/interfaces/05-composition/book.go b/interfaces/06-composition/book.go
similarity index 100%
rename from interfaces/05-composition/book.go
rename to interfaces/06-composition/book.go
diff --git a/interfaces/05-composition/discount.go b/interfaces/06-composition/discount.go
similarity index 100%
rename from interfaces/05-composition/discount.go
rename to interfaces/06-composition/discount.go
diff --git a/interfaces/05-composition/game.go b/interfaces/06-composition/game.go
similarity index 100%
rename from interfaces/05-composition/game.go
rename to interfaces/06-composition/game.go
diff --git a/interfaces/05-composition/list.go b/interfaces/06-composition/list.go
similarity index 100%
rename from interfaces/05-composition/list.go
rename to interfaces/06-composition/list.go
diff --git a/interfaces/05-composition/main.go b/interfaces/06-composition/main.go
similarity index 100%
rename from interfaces/05-composition/main.go
rename to interfaces/06-composition/main.go
diff --git a/interfaces/05-composition/puzzle.go b/interfaces/06-composition/puzzle.go
similarity index 100%
rename from interfaces/05-composition/puzzle.go
rename to interfaces/06-composition/puzzle.go
diff --git a/interfaces/05-composition/time.go b/interfaces/06-composition/time.go
similarity index 100%
rename from interfaces/05-composition/time.go
rename to interfaces/06-composition/time.go
diff --git a/interfaces/05-composition/types.go b/interfaces/06-composition/types.go
similarity index 100%
rename from interfaces/05-composition/types.go
rename to interfaces/06-composition/types.go