Today’s Wonder of the Day was inspired by Rowan. Rowan Wonders, “What is fermentation?” Thanks for WONDERing with us, Rowan!

Do you like pizza? Of course, you do! Who doesn't, right? Is there anything that smells quite as fabulous as a piping-hot pepperoni pizza fresh out of the oven?

The tomato sauce blends with the melted mozzarella cheese. The garlic and fresh-baked crust give off aromas that will make your mouth water.

What other smells make your nose happy? For some people, there's nothing like the smell of a pine forest just after a spring rain.

Others might love to stop and smell the roses when they're in bloom. If you ever visit a candle store, you'll quickly realize there's no end to the types of things that smell good.

Of course, there are also plenty of things that don't smell so good: your socks after soccer practice, garbage cans, dirty diapers, stinky cheese, rotten eggs, skunks, etc. The list goes on and on.

What's the worst thing you've ever smelled in your life? If you had to narrow it down to the most objectionable odor ever, what would it be?

For some people, there's a clear answer to the worst smell you can imagine: surströmming. Say what? Don't worry if you've never heard of surströmming. Most people outside of Sweden have never heard of it…or eaten it.

That's right! Surströmming is a food. Specifically, it's a type of fermented herring. The name surströmming is Swedish for "sour herring."

In May of each year, Baltic herring are caught just prior to spawning. They are placed into a strong brine solution for about a day to draw out their blood. The fish are then gutted and their heads are removed before being placed into a weaker brine solution.

In the brine solution, the process of fermentation begins. A lactic acid enzyme in the spines of the fish releases foul-smelling acids and hydrogen sulphide. As they continue to stew in their own bacteria, just enough salt is added to keep them from rotting.

Although this process might sound a bit unappetizing, it's actually a food preservation method that has been around for centuries. At the beginning of July, the fish is canned for sale beginning the third Thursday in August.

The third Thursday in August marks the traditional time to have a surströmmingsskiva, which is a party centered around opening cans of and eating surströmming. While not everyone in Sweden likes surströmming or celebrates it with a party, the stinky fish is particularly popular in northern Sweden.

If you buy a can of surströmming, you need to keep it in a refrigerator. The process of fermentation continues in the can, and cans will bulge noticeably over time as the canned fish ages. If it gets too warm, lactic acid will destroy the fish.

When it's time to crack open a can of surströmming, experts recommend that you do it outside. Some even open the cans underwater to minimize the unpleasant smell.

What does it smell like? More pungent than normal rotting fish, some have compared the smell to dead bodies, dirty diapers, stinky cheese, vinegar, and seafood all wrapped into one horrible smell!

Thankfully, surströmming is not supposed to eaten by itself. Instead, chefs recommend eating it as part of a sandwich on thin, buttered bread and topped with thinly-sliced potatoes, diced red onion, sour cream, chives, and dill. Some people drink milk and eat a wedge of hard cheese along with their surströmming sandwich!

Wonder What's Next?

Tomorrow’s Wonder of the Day takes a closer look at a mysterious condition that affects millions of people around the world!