Clicking on an object will shrink it; hold space and click and you can expand another object with the accumulated mass.
So, for example, in the pic above, the purple ball rolls slowly along the slight incline of the brown platform. It will eventually roll off, land on the yellow bridge and continue rolling to the left.
However, if I click on the small yellow block on the right, thus shrinking it, the yellow bridge will tilt and the purple ball will land and then roll right. It'll hit the smiley face and push it into the level exit (the white rectangle).
Later levels get more complex as you're transferring mass all over the place and it really starts to tax your brain. It's really very clever.
Shrink It [Armor Games]