Interesting concept! Here's some feedback:
When I first started I had 0 ideas what to do. I had to read the page in order to know what exactly the goal was, try to have this in-game next time :)
Then the mechanic itself by throwing rocks, should be more polished. It didn't really feel that good because:
It was difficult to pick up the rocks as the hand would collide into the wall.
Because the hands collided with the wall, any rocks that dropped from within becomes impossible to pick up.
What I'm trying to say here, remove the hand's collider :P