Lisa Ehglmeier, age 11, of Florence, South Carolina, for her question:

How do the holes get into Swiss cheese?

Those impossible empty round holes in a solid slab of Swiss cheese are called eyes. They are put there by P. helveticum, a scientific name which tells us absolutely nothing. However, the P. stands for Propionibacterium  and the bacterium part of this word gives us a clue to what goes on behind the scenes. After all, most of the work of cheesemaking is done by tiny bacteria, too small for human eyes to see.

To make any kind of cheese, the milk is curdled and colonies of bacteria are allowed to digest the solids. When left in a cool, quiet place, different strains of bacteria transform the milk into cheeses of various textures and flavors. During this digestive process, the bacteria that make Swiss cheese create gases. These gases collect in round bubbles and, as the maturing cheese grows solid, the bubbles cannot escape. This is how those empty round holes get into Swiss cheese.