We were looking for some cheddar cheesein the Wonderopolis refrigerator the other day when we overheard the mayonnaise joking with the mustard.

Mayonnaise: What do you call cheese that is sad?

Mustard: I have no idea.

Mayonnaise: Blue cheese!

Mustard: Don't quit your day job.

Have you ever eaten blue cheese? It's often crumbled onto salads or made into salad dressings. It has a unique smell that some people like and others don't. Maybe its odor offends the other inhabitants of the refrigerator and that makes it sad!

It's not the blue cheese's fault, though. That's just the way that it's made. Blue cheese — sometimes called bleu cheese (French spelling) — is cheese made from the milk of cows, sheep or goats with the mold Penicillium added.

This special mold creates the unique veins of blue or blue-green mold throughout the cheese. It is these blue veins in the cheese that gave it its name, as well as its signature sharp and salty flavor.

These veins of mold, along with certain types of bacteria, also give blue cheese its special smell. For example, one of the types of bacteria common in blue cheese — Brevibacterium linens — is the same bacteria behind foot and human body odor!

Instead of an invention, blue cheese is thought to have been more of an accident. Over a thousand years ago, cheeses were aged in caves, because their environments were cool and moist. The harmless molds in these caves made their way into aging cheeses and blue cheese was accidentally discovered. Today, many blue cheese makers still age their blue cheeses in caves.

Blue cheeses are unique in appearance because of the veins of mold that run throughout them. These veins are created by a process called needling. Wheels of cheese are poked with tiny needles to create spaces for air. As air finds its way to these holes, it feeds the mold and the blue veins slowly form.

Of course, not all blue cheeses smell and taste the same. Like any cheese product, there are many factors that affect the final product, including the type of milk used, the exact mixture of molds and bacteria and the aging process. Many types of blue cheeses from around the world are considered delicacies.