I've become engrossed in this fascinating game which defies the laws of physics.
The object of the game is to leave all the green elements on screen while removing all the red or pinkish coloured ones. The purple ones are neutral.
There are several sites hosting the game with various configurations, but I completed all 40 levels in this one last night: Red Remover #1
And all 40 games on this one today: Red Remover #2
The first half dozen or so in both are child's play, but it gets more and more difficult the higher you progress up the scale.
Definitely recommended for those who enjoy a challenge!
EDIT: I should add that you don't have to signup to play on either site.