Drop9 is this beautiful and delightful game that will give your brain some stimulation as you play.
The objective of Number III is to click on 3 falling numbers to combine them, but to progress in the game you will need to create sums of 0, 10, and 20. Sums lower than 10 simply combine to form a new droplet containing the sum; but sums higher than 10 are marked "over" and reduce the 'health' bar at the bottom. Letting the droplets escape beyond the bottom dotted line also reduces health. If the heath bar depletes entirely, it's game over.
Try to achieve the highest score you can. A nice point bonus is awarded for combining 3 zeros.
It's a very simple (and short) game that is just right for an afternoon delight.