Play ball with your dog, without you!
If you have really energetic dogs at home, but you never have time to play with them, or you're too tired, this is perfect for you. It is a machine that plays fetch with your dog(s). Place it in a corner of your yard, put a time you want it to play, then let it loose. It has 10 tennis balls in a compartment inside it. Inside those balls are powerful magnets that get attracted to the machine when the machine runs out of balls. Like the machines you see in tennis courts, it will throw the balls in many directions at intervals of 20 seconds, plenty of time for your dog to chase them. Then it will neatly pick the balls up, leaving your yard balls-free.
Reward: get it for free, the ability to use it in a science fair without getting sued.