You can't always stick around to keep your pet's water bowl full, but you can build a clever contraption that will do the job for you. You just need a two-litre bottle, some wood and a few tools.
Instructables user JimTiffinJr's 11-year-old daughter came up with this clever project. How does it work? You line the bottle up with the inside of the bowl so once it becomes submerged you won't have any more water flow out automatically. When the dog or cat drinks the water and the cap is no longer submerged, more water will come out.
You can make this easily, and it works well without any complicated mechanisms.
The Awesomest Coolest Easiest Water Bowl for DOGS & CATS! [Instructables]