Mark Wiles, age 9, of High Point, North Carolina, for his question:

How do those round holes get into Swiss cheese?

Human beings do not make those impossible holes. They are made from inside the solid cheese. And even the smallest person is much too big for this job. The work is done by tiny tiny living things. But don't look for them inside the round bubbles. Even if they were still there, they are much too small for your eyes to see.

The tiny living things that make the holes in Swiss cheese have very big names. Scientists call them propionibacteria. You will notice that the last two pieces spell out the word bacteria, and you may not like that word because some bacteria make us sick. However, this is hardly fair because most bacteria do not harm us at all.

We share our world with countless numbers of bacteria. And there are thousands of different kinds. It takes a row of 2,500 of them to measure about one inch. We cannot see them unless we magnify them under a microscope. They are everywhere around us  ¬in the ground and in the water, floating in the air and on everything we touch. Bacteria share our world and we cannot escape them. It is true that a few types give us infections and diseases. But most types do not bother us. And a few types are very friendly and helpful to us.

For example, our cheeses are made by helpful bacteria. Cheese, as you know, is made from milk. Different types of bacteria change the milk into different kinds of cheese. The propionibacteria types create Swiss cheese and also put in those mysterious round holes. When a dairy makes Swiss cheese, a small helping of these very special bacteria is added to the milky recipe.

First the milk is curdled. They separate the solid particles that float in the watery liquid. The milk solids clot together in soft gobs. These are scooped out and drained. This mixture is set aside. It is left quietly alone while the bacteria do their work. They feed and feed, thrive and multiply. As they do these things, the milky mixture is changed. Some of it becomes invisible gas and some becomes solid cheese. The gases collect together in bubbles. They cannot escape because the solid cheese sets around them. At last the job is done and the cheese goes to market. The bubbles of gas are still waiting there, trapped inside. They are the round holes that surprise you when you cut a slice of Swiss cheese. The propionibacteria that created them added more pleasant surprises to the recipe. They gave the cheese its special  tangy flavor. And best of all, they also added a lot of body building nourishment to the recipe.

For ages, Swiss cheese was made only in Switzerland. This is where the original propionibacteria lived    hiding in the milk, in the dairies and everywhere else. Nobody knew about bacteria until the microscope was invented. Then scientists spotted the tiny Swiss cheese makers. They captured them and separated them from other bacteria. Then they sent samples to other countries. This is why our dairies can make the special cheese that once was made only in Switzerland.