diff --git a/mocking/countdown.go b/mocking/countdown.go
new file mode 100644
index 0000000..a3e4e8b
--- /dev/null
+++ b/mocking/countdown.go
@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"time"
+)
+
+type Sleeper interface {
+  Sleep()
+}
+
+type SpySleeper struct {
+  Calls int
+}
+
+func (s *SpySleeper) Sleep() {
+  s.Calls++
+}
+
+type DefaultSleeper struct{}
+
+func (d *DefaultSleeper) Sleep() {
+  time.Sleep(1 * time.Second)
+}
+
+const finalWord = "Go!"
+const countdownStart = 3
+
+func Countdown(out io.Writer, sleeper Sleeper) {
+  for i := countdownStart; i > 0; i-- {
+    fmt.Fprintln(out, i)
+    sleeper.Sleep()
+  }
+  fmt.Fprint(out, finalWord)
+}
+
+func main() {
+  sleeper := &DefaultSleeper{}
+  Countdown(os.Stdout, sleeper)
+}
diff --git a/mocking/countdown_test.go b/mocking/countdown_test.go
new file mode 100644
index 0000000..231f5d6
--- /dev/null
+++ b/mocking/countdown_test.go
@@ -0,0 +1,27 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestCountdown(t *testing.T) {
+  buffer := &bytes.Buffer{}
+  spySleeper := &SpySleeper{}
+
+  Countdown(buffer, spySleeper)
+
+  got := buffer.String()
+  want := `3
+2
+1
+Go!`
+
+  if got != want {
+    t.Errorf("got %q want %q", got, want)
+  }
+
+  if spySleeper.Calls != 3 {
+    t.Errorf("not enough calls to sleeper, want 3 got %d", spySleeper.Calls)
+  }
+}