Steven Gustafson, age 9, of Washington, Illinois, for his question:

How do holes yet into Swiss cheese?

Almost everybody is puzzled by this mystery. Many pen pals ask Andy to explain it, but only one can be selected for the column. The answer is meant for everyone who is interested in this topic. If you happened to ask the same question, you are smart enough to think of ten more, so try 'again. There is no limit to how many questions you may ask Andy    and who knows, next time around he may select one of yours to appear in the column.

The holes is Swiss cheese look like bubbles because they are bubbles. This seems odd because most bubbles are frothy little balloons that soon burst and disappear in the air. At least, this is what happens to the sparkling ones in pop and the foamy ones that belong to the soap. Those Swiss ones seem strange because they are surrounded by solid cheese. When we slice through a chunk of the cheese, we cut right through them and they look like smooth round holes.

The mystery is how anybody can put those beautiful bubbles inside the solid cheese. Actually, the magic is not done by human hands. The magicians are the same little busybodies who turned the original milk into cheese. They created the bubbles as they did their work    and to them this was no trouble at all. Swiss cheese is made by bacteria that are much too small to be seen. It takes about 12,000 of them to make a row as long as your thumb nail.

There are thousands of different bacteria, but only a couple of them can make Swiss cheese. Scientists gave them long fancy names, perhaps to make up for their small sizes. One is Propionibacterium helvetium, the other is Propionibacterium shermanii. These two strains of bacteria are carefully cultivated and greatly treasured. Cheese makers add a small helping of one of these cultures when they want to make Swiss cheese with proper round holes and the right tangy taste.

The basic ingredient is milk. First they pasteurize it to destroy the useless and harmful bacteria. Then they add an acid to curdle the milk and scoop out the solid clots from the liquid. The bacterium culture is added to the clotted milk solids and the mixture is set in a cool place to ripen.

The busy bacteria set to work on the milky food. As they digest it, it changes  into cheese and gas. The gas collects together in big bubbly holes and the cheese sets around them. It takes a long time to use up all the milky material. When the job is done, those round holes are trapped and sealed inside the solid cheese.

There is no way to remove the bacteria from the finished cheese. But this is no cause for alarm. They happen to be quite harmless types and also very nourishing. Actually they teem with rich proteins that help a person's muscles to grow strong. This is one reason why sensible folk eat lots of cheese    with or without those smooth round holes.