Is the most awesome game I have played in a very long while. It is cute, stylish, simple yet effective and even emotionally affecting at times.
How to explain it...? You can control little sentient balls of goo that can join to form structures (if you've played the indy game Tower of Goo, you've got the basic premise). You have to build structures to allow the goos to cross the level and get out of the other side. Except you're building it out of goo, so the more complicated the structure, the fewer goos you can save. And there are many different types of goo, so the gameplay is affected depending on what is available.
I'm not able to sum up in words (
goo Goo GOO) how awesome this game is. Take a look at
this review from Eurogamer; possibly the most positive review I've read of any game ever. Then
download the demo from here (it's the entire first world of the game). Then buy it either from the developers' website or from Steam. Then see how often you can fit the word 'goo' in to normal conversation before people start suspecting you're insane.