The game is very intuitive --there are 20 number tiles, and you must click them to add up to the required number (which changes with each turn). Each number tile features a single-digit number, and the sums you must make are anywhere from 1 to 27. When you use a tile, it disappears from the board--your goal is to clear the board as fast as possible. There's a timer, a move counter (which counts the number of moves it takes you to clear the board), and a progress bar (which gives you a visual on how many tiles you need to clear the board). If you accidentally go over the required sum with your tiles, the board flashes red (at the end of your game, you'll get an accuracy reading).

There is a two-player option, in which you can race against a friend. The two-player mode features two side-by-side mini Numbl boards, and each player sits opposite each other and tries to add up the numbers on their side of the board as fast as they can. It is a race--not timed--so whoever beats their board first, wins. Numbl does keep track of each player's wins, so you can have a mini Numbl "tournament," if you so desire.

There is an online leader board, so you can see how you're doing compared with everyone else in the world (you can check out high scores of all time, the month, the week, and the day). There's also a high score list for your iPhone, if you just want to see how you're doing.

The interface is worth mentioning--it's gorgeous. It's very simple and elegant in blue and black, and there's no clutter or excess of features on the screen. In each new game, the number tiles cascade from the top onto the board, and the board itself features a modern number pattern. The sounds--there's no music, just simple sound effects--are also simple and succinct.

Numbl is, honestly, pretty addictive--even if you're just racing against yourself. The only issue I found was that the games end up being pretty short, especially as you get better. The two-player mode, however, is a nice touch. Just make sure you're playing against someone with comparable math ability or it get boring quickly.