According to the title, The Walls Are Not Cheese. I don't know what they are, exactly, but they do exhibit a slight cheese-like characteristic, specifically in that they crumble when you shoot them. That's just one of the fun game mechanics in this deceptively simple game. It's only a few short levels, so you should be able to finish it in less than ten minutes, but it's still worth checking out. Oh, and if you have a tiny monitor (like me) you can hit F11 to go full-screen so you can see the whole thing.