Very polished, great concept. Only gripes I have:
I wish you could turn the block a bit faster when dragging the mouse.
I found nail a few times in the tools level, but it wouldn't register until I found the "proper" nail.
Did the game mention anywhere that words could be backwards? The level "Tools" took me a bit without realizing that, and animals had one of those too.
Overall, this is fantastic. I can very easily see this being a marketable product with a little more polish.