Cracks on Cover

• Cracks in the plastic of the cover, as long as they're small and not opening up the inner workings to damage, usually allow the Kindle to work. You must be extra careful with exposure to the elements or dirt since water or particles inside the Kindle could prevent it from working.
  
  

Cracks on Keys

• Cracks on keys are more likely to cause a problem. It's possible for a small piece of broken plastic to get stuck and get in the way of other internal workings. The most common thing is for that key to simply stop working. As long as it's not a key you use often, you might find this acceptable.
  
  

Cracks on Screen

• Cracks on the screen are the most likely to prevent you from using your Kindle, especially if they are large. As long as the Kindle's internal workings are safe, the e-books will still show up on screen in most cases. However, a large crack can mean that you lose letters or even entire words under the crack, making reading difficult.
  
  

Fixing Cracks

• Kindles come with a "One-Year Limited Warranty" according to Amazon. For Kindle 1 and Kindle 2, your device is probably not covered anymore. Amazon's policy on cracked Kindles is to replace them: they currently don't offer repair services for cracks. Find out if your Kindle is covered under warranty by calling Amazon customer service at (866) 321-8851.