diff --git a/pic/pic.go b/pic/pic.go
index 21edae7..7f22edd 100644
--- a/pic/pic.go
+++ b/pic/pic.go
@@ -1,7 +1,9 @@
-// Copyright 2011 The Go Authors.  All rights reserved.
+// Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Package pic implements functions that
+// display pictures on the Go playground.
 package pic // import "golang.org/x/tour/pic"
 
 import (
@@ -12,7 +14,16 @@
 	"image/png"
 )
 
-func Show(f func(int, int) [][]uint8) {
+// Show displays a picture defined by the function f
+// when executed on the Go Playground.
+//
+// f should return a slice of length dy,
+// each element of which is a slice of dx
+// 8-bit unsigned int. The integers are
+// interpreted as bluescale values,
+// where the value 0 means full blue,
+// and the value 255 means full white.
+func Show(f func(dx, dy int) [][]uint8) {
 	const (
 		dx = 256
 		dy = 256
@@ -32,6 +43,8 @@
 	ShowImage(m)
 }
 
+// ShowImage displays the image m
+// when executed on the Go Playground.
 func ShowImage(m image.Image) {
 	var buf bytes.Buffer
 	err := png.Encode(&buf, m)