The holes are called eyes, and they're made of gas. As Swiss-style cheese ripens, bacteria in it (called propionic bacteria) eat lactic acid and give off carbon dioxide. The gas collects into bubbles, and because the cheese is firm, the bubbles stay put and become holes.

The surprising part: researchers in Switzerland found that the bubbles need something to start forming around, and tiny particles of hay that fall into the milk do the job. Add more hay dust to the milk and you get more holes. That also explains a modern change: sealed, automated milking keeps hay dust out of the milk, so many Swiss cheeses now have fewer and smaller holes than they used to.

And no, it isn't mice.

If you like the holey look, Jarlsberg is a Swiss-style cheese with big, well-developed eyes.