Long ago, ancient Romans placed stone pillars called "obelisks" along the sides of roadways. Typically, the stones were placed a mile apart. Each “mile stone" was given a unique number, serving as a mile marker.

Today most people are familiar with the word “milestone," but we use it in a slightly different way. Our modern-day use was inspired by the Romans' ancient practice. Just as Roman mile stones helped travelers know how far they had come on their journey, our modern use of this expression does the same thing.

Instead of marking our journey on an actual road, however, milestones mark significant events in our lives. People reach milestones throughout their lives. You may not realize it, but you have already reached many milestones in your life, too.

As a baby, you reached milestones when you learned how to roll over, crawl, stand up and, finally, walk. Children celebrate many milestones as they grow up: learning to read, riding a bicycle without training wheels, getting — and losing — your first tooth and going to school for the first time are just a few examples.

You've got your whole future ahead of you — and that means many more milestones ahead! Graduating from high school or getting your driver's license will be major milestones. If you ask your parents, they're sure to tell you their wedding day and the births of their children were happy and important milestones in their lives.

Wonderopolis is celebrating a WONDERful milestone today: This is our 100th Wonder of the Day!

What makes the number 100 so special? If you pay attention, you will find 100 popping up everywhere from stove tops to piggy banks. As the first three-digit whole number, 100 is considered important around the world.

Water boils at 100° C. There are 100 pennies in a dollar. A century is 100 years. If something is 100 percent, it's considered complete or full.

People have given special recognition to 100 to mark achievements or accomplishments for hundreds of years. From 100th anniversary celebrations to 100th birthdays, 100th episodes to 100th wonders, 100 marks a milestone — and we're just getting started!

Over the past 100 days, we have explored why flamingos are pink, how horses sleep standing up and what bees do in the winter. We've learned why chili peppers are spicy and why cheddar cheese is orange.

We've taken flight with the Wright Brothers, Amelia Earhart and a flock of birds in a flying “V." We've jiggled Jell-O, twisted fairy tales and invented superheroes to save the world.

We're so glad you've joined us to celebrate our 100th Wonder of the Day milestone. We hope you will take some time to explore and reexplore Wonders of the Day #1 to #99.

Do you have a favorite? Send us an email, post a comment in the section below or send us a message on Facebook or Twitter to let us know which Wonder of the Day is your favorite so far and why.

As for Wonders of the Day #101 to #200, we want to know what you've been wondering about. Would you like to find out what a narwhal is or why bats sleep upside down? Maybe you've been wondering how boats stay afloat or how glass is made.

Tell us what questions are on your mind! Your curious question could inspire a future Wonder of the Day.

Wonder What's Next?

Today’s wonder took you back in time; tomorrow’s wonder will take you forward and backward in the same sentence. It may even leave you asking, “Was it a car or a cat I saw?”