Free Kindle Books for Kids: Hilarious Knock Knock Jokes for Kids, Sheep Don’t Whistle March 19, 2012 by Two funny books on our list of FREE Kindle Books for Kids today: Hilarious Knock Knock Jokes for Kids Sheep Don’t Whistle If you don’t have a Kindle you can still read this on your computer, iPad, iPod, iPhone, or Android. Remember that Amazon changes prices often so this may not be free for long.