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Tetris meets Sudoku in this original, addictive puzzle game.

Before you start playing it, you’ll think this is a pretty simple variation on the classic Bejeweled clones — replace symbols with numbers, woo!

But, as you’ll quickly find out when you start playing it and have the hardest time figuring out how to actually play it right, this is one of the most creative takes on “drop pieces in a grid” I’ve ever seen. And, like all the best games, it’s got incredibly simple mechanics, but they result in a deep and highly addictive game.

Here’s the basic principle: a block marked with a number disappears from the grid when it’s part of a group containing that number of blocks. So, if you have a full row of seven blocks, containing a couple 7s, only those 7s will disappear. If you have a column of four pieces with a 4, a 3, a 2, and a 1 (in any order), then the 4 will disappear; then the 3 will disappear, then the 2, then the 1 (and you’ll get lots of points for that, because that’s a x4 chain).

In addition, because that would probably be a bit too simple, the game throws unmarked, sealed blocks at you, that will only reveal the number they contain after they’ve been near a number that disappeared from the grid. And for each new level, a whole row of those mystery discs pops up from the bottom of the grid — dooming you if you happen to be stuck with a bunch of 1s on top of them.

I don’t think it’s possible to explain why the game is so cool (which is why I’m not trying very hard), so if you’re at all into puzzles with falling pieces, and aren’t afraid of counting (unlike Sudoku, Drop7 is indeed mathematical), you absolutely have to download the free edition (iTunes link). And you may well not feel the need to upgrade to the paid version (it doesn’t look like the Lite game is limited in any way that really matters), but you should, just because it’s such a clever puzzle game.

Two strategy hints to get you started:

• your very first priority should be to get rid of 1s, almost at any cost

• the second most important thing in the game is to uncover the numbers hidden in those gray discs, so any turn that didn’t contribute to chipping away at one of those discs is a lost turn (and any lost turn gets you closer to the next level-up with its whole additional row of gray discs)

Kiss your productivity and your battery good-bye (I’m not sure such a game should drain the battery that fast, but then almost every app does nowadays). And good luck getting rid of your deeply-ingrained Tetris or Bejeweled reflexes, they won’t help you at all here.

Well worth the price.