The knife on the plate represents 0 – it is a balanced budget hence there is no cake.
Here we have a surplus as there is cake above the knife. This cake can be stored for a rainy day so that everyone can enjoy it.
Here we have a deficit as there is cake below the knife. This cake must be cut and eaten right away. It won’t be pleasant but you’ve got to do it. You’ll feel ashamed and fat after you’ve eaten it, but you know that you’re on the way to a surplus.