diff --git a/countdown/countdown.go b/countdown/countdown.go
index 4cd054b..89bc546 100644
--- a/countdown/countdown.go
+++ b/countdown/countdown.go
@@ -11,11 +11,22 @@ const (
 	countdownStart = 3
 )
 
-func Countdown(out io.Writer) {
+type Sleeper interface {
+	Sleep()
+}
+
+type DefaultSleeper struct{}
+
+func (d *DefaultSleeper) Sleep() {
+	time.Sleep(1 * time.Second)
+}
+
+func Countdown(out io.Writer, sleeper Sleeper) {
+	// XXX: The sequence is not tested
 	for i := countdownStart; i > 0; i-- {
 		fmt.Fprintf(out, "%d\n", i)
 		// This is difficult to test!
-		time.Sleep(1 * time.Second)
+		sleeper.Sleep()
 	}
 	fmt.Fprint(out, finalWord)
 }
diff --git a/countdown/countdown_test.go b/countdown/countdown_test.go
index 524b01c..c55330e 100644
--- a/countdown/countdown_test.go
+++ b/countdown/countdown_test.go
@@ -5,10 +5,19 @@ import (
 	"testing"
 )
 
+type SpySleeper struct {
+	Calls int
+}
+
+func (s *SpySleeper) Sleep() {
+	s.Calls++
+}
+
 func TestCountdown(t *testing.T) {
 	buffer := &bytes.Buffer{}
+	sleeper := &SpySleeper{}
 
-	Countdown(buffer)
+	Countdown(buffer, sleeper)
 
 	got := buffer.String()
 	want := `3
@@ -19,4 +28,8 @@ Go!`
 	if got != want {
 		t.Errorf("got %q want %q", got, want)
 	}
+
+	if sleeper.Calls != 3 {
+		t.Errorf("not enough calls to sleeper, want 3 got %d", sleeper.Calls)
+	}
 }
diff --git a/main.go b/main.go
index f97a750..2841216 100644
--- a/main.go
+++ b/main.go
@@ -8,5 +8,5 @@ import (
 func main() {
 	// greet.Greet(os.Stdout, "Elodie")
 
-	countdown.Countdown(os.Stdout)
+	countdown.Countdown(os.Stdout, &countdown.DefaultSleeper{})
 }