diff --git a/src/study/day7Function/DeferredFunc.go b/src/study/day7Function/DeferredFunc.go
index 791863f..ca026e7 100644
--- a/src/study/day7Function/DeferredFunc.go
+++ b/src/study/day7Function/DeferredFunc.go
@@ -24,7 +24,7 @@ func title(url string) error {
 	}
 	// 直到包含defer的函数执行完毕之后，defer的这条语句才会执行
 	defer resp.Body.Close()
-	// Check Content-Type is HTML (e.g., "text/html;charset=utf-8").
+	// Check Content-Type is Html (e.g., "text/html;charset=utf-8").
 	ct := resp.Header.Get("Content-Type")
 	if ct != "text/html" && !strings.HasPrefix(ct, "text/html;") {
 		return fmt.Errorf("%s has type %s, not text/html", url, ct)
@@ -32,7 +32,7 @@ func title(url string) error {
 	doc, err := html.Parse(resp.Body)
 
 	if err != nil {
-		return fmt.Errorf("parsing %s as HTML: %v", url, err)
+		return fmt.Errorf("parsing %s as Html: %v", url, err)
 	}
 	visitNode := func(n *html.Node) {
 		if n.Type == html.ElementNode && n.Data == "title" && n.FirstChild != nil {
diff --git a/src/study/day9Interface/main.go b/src/study/day9Interface/main.go
new file mode 100644
index 0000000..836fe28
--- /dev/null
+++ b/src/study/day9Interface/main.go
@@ -0,0 +1,22 @@
+package main
+
+import "fmt"
+
+type www interface {
+	Write()
+}
+type ByteCounter int
+
+func (c *ByteCounter) Write(p []byte) (int, error) {
+	*c += ByteCounter(len(p))
+	return len(p), nil
+}
+
+func main() {
+	var c ByteCounter
+	c.Write([]byte("hello"))
+	fmt.Println(c)
+	name := "dugulp"
+	fmt.Fprintf(&c, "Hello, %s", name)
+	fmt.Println(c)
+}