If you have any common sense, you'll know that looking at any screen for very long periods of time regularly can destroy your eyes. You won't notice any effects at first but over the years there will be problems. So why not remind yourself to take a break with this very helpful Chrome extension. 20 Cubed reminds you every 20 minutes to take a 20 second break and look at something 20 metres away. It's literally as simple as that. You'll thank yourself when you're old.