Sidewalks are great places to play, aren’t they? If you want to ride your bike, a smooth sidewalk is much better than a busy street or a bumpy lawn. If you prefer to play jacks or hopscotch, where do you go? The sidewalk, of course!

Most sidewalks are made of concrete. Concrete is not a natural element, like aluminum or iron. Instead, concrete is a man-made building material.

Concrete is made by combining crushed rock and sand with water and cement. Although some people think cement is the same thing as concrete, it’s not.

Cement is a general name for a special type of material that binds other materials together. In other words, cement is like another name for glue. There are many different types of cements that can be used to make many different types of concrete.

When cement is mixed with water and crushed rock and sand, a chemical process called hydration causes it to bond together and harden to form concrete. Solidified concrete is like stone, and it’s used more than any other man-made building material in the world.

One of the uses of concrete is to make sidewalks. Unfortunately, when concrete dries, it shrinks a little bit. If you’re using concrete to make a sidewalk, the shrinking concrete will cause cracks to appear as it dries.

To prevent sidewalks from cracking in random spots and breaking apart, builders make lines in sidewalks. Of course, they don’t just call them lines. The technical term for sidewalk lines is contraction joints.

Contraction joints are placed in fresh concrete before the concrete dries and has a chance to create its own joints, which we call cracks. As the concrete dries and shrinks, any cracks that form should follow the path of the contraction joints, since that’s where the concrete is thinnest.

If the cracks follow the contraction joints, the sidewalks won’t look as bad. They also won’t tend to form further cracks and break apart like they might if allowed to crack at random places.

Builders place contraction joints in fresh concrete with saws, special grooving tools or plastic strips called zip-strips. They have to make sure that the joints are deep enough and made before the concrete begins to dry, so that cracks won’t appear in the wrong places.

28 Join the Discussion

1 Star2 Stars3 Stars4 Stars5 Stars  (15 votes, avg. 4.27 out of 5)
    • That’s a GREAT guess about tomorrow’s Wonder, Missy! You’ll have to check back to see if you were right! Thank you for hanging out in Wonderopolis today and leaving us a comment! :-)

    • We like the video, too, Natalia! That little boy sure does like to hop a lot, doesn’t he? Thank you for visiting Wonderopolis today! :-)

  1. Dear Wonderopolis,

    We loved the article. Tomorrow, for Poetry Friday, we are going to write sidewalk poems. Then we will go outside and write our poems on the sidewalk. We hope it won’t be raining.

    Have you ever heard the saying, “Don’t step on a crack or you’ll break your mother’s back?” Do you know where that saying came from?

    Thanks again!
    Mrs. Johnson’s third grade

    • We are so excited to hear about your sidewalk poems, Mrs. Johnson’s third grade! We will send good wishes for great weather! We have heard the saying, “Don’t step on a crack or you’ll break your mother’s back,” but we’re not sure of its origin. That means it’s time for some more WONDERING! Thank you for leaving us this great comment today! :-)

  2. I actually never thought about how a sidewalk got its cracks, I figured it just happened! Sometimes, my friends and I would try to not walk on the cracks or the “contraction joints” while we walked across the sidewalks. I think I probably should pay a little more attention to the stuff about sidewalks, like the difference between concrete and cement, the reason the lines are there in the sidewalk and where the cracks come from. This wonder has really made me wonder!

    • We’re glad exploring this Wonder caused you to do a little extra WONDERing, Allison! It’s always good to take what you learn about something and strive to learn more, more, MORE! Thank you for leaving us this great comment today! :-)

  3. This is a wonderful wonder, because I’ve never thought about it before. It is more interesting then I thought it would be. I never knew sidewalk cracks actually had a name, contraction joints. Also, I always thought cement and concrete were the same thing. I wonder what people would do if cracks already were there in a sidewalk. I think the next wonder is about memory loss.

    • We thought it was interesting to learn new things about sidewalks, too, Jenna! Thank you for sharing this comment with everyone in Wonderopolis today, and also for guessing what you think tomorrow’s Wonder might be about! We can’t wait to see if you are right! :-)

  4. Hey, WONDEROPOLIS! I think that your next wonder should be, “Why is the sun yellow?” By the way, I thought today’s wonder was pretty interesting.

    • Those are all GREAT ideas for future Wonders of the Day®, Natalia! Thank you for suggesting them and for being an AWESOME Wonder Friend! :-)

  5. I thought this wonder was very interesting! I thought cement and concrete were the same thing. I thought cracks or “contraction joints” just happened because the sidewalk was old or worn. Now I know that when concrete dries, it cracks and that’s why there are cracks on the sidewalk. I also thought it was interesting to learn what is in concrete and how it is made. I used to play hopscotch on the sidewalk when I was little, and I still like to rollerblade or ride my bike on it, too. I wonder what tomorrow’s wonder will be?

    • We’re sure glad you learned all these awesome facts about sidewalks, Anne! It’s pretty neat to learn new things every day in Wonderopolis, isn’t it? We really enjoyed your comment! :-)

  6. Wonderopolis,
    This is a really great wonder! I really enjoyed learning about sidewalks! I never knew that cement and concrete were not the same thing! I also never knew that cement is not a natural element! To go even farther, I wonder why sidewalks are mostly always white? Thank you so much for having this wonderful website and wonderful topics!!

    • Your comment made our day, Team Unger #2! We’re so glad you think Wonderopolis and the Wonders of the Day are WONDERful! Thanks for stopping by this Wonder about sidewalks…we’re glad you learned a lot! :-)

    • Thanks so much for your comment today, Sherry! We’re super excited to hear from Wonder Friends who are also awesome educators!

      Because of the age range of our different Wonder Friend visitors, and for the benefit of everyone who visits Wonderopolis and gains even MORE knowledge by reading over the comments to each Wonder, we occasionally have to adjust the spelling of a word or two, or help by adding punctuation. We try very hard not to disturb the integrity of each comment’s content, though. We value every authentic comment, and try our best to reply to each one. We’re glad you stopped by Wonderopolis today! :-)

  7. Call for stroller-friendly sidewalks:

    That explains the line.
    But what about the annoying slight bump they build on either side of the line? Makes for a bumpy baby’s stroller ride. If babies had their say…

    • Hey there, Rs11, we are glad you’ve been thinking about how sidewalks impact others! :)

      Baby strollers probably do feel the bumps, cracks and ups and downs of the sidewalk, great point! Perhaps a smooth running path is a better route for a long walk! :)

    • Thanks for sharing your comment with us, Emma! We are thrilled that you learned something new with us about the sidewalks we use every day! HOORAY for WONDERing! :)

    • Way to go, Henry, we’re so glad you learned something new with us today! Now you can Wonder as you travel along your neighborhood’s sidewalks! Thanks for telling us how much you enjoyed the video, too! We’re glad to have an awesome Wonder Friend like you! :)

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>


  • Wonderopolis on Facebook
  • Wonderopolis on Pinterest
  • Print

Have you ever wondered…

  • Why do sidewalks have lines?
  • What is a contraction joint?
  • What is concrete made of?

Wonder Gallery

weed growing crack_shutterstock_60868711Vimeo Video

Try It Out

When you walk along the sidewalk, do you try to avoid the lines? Isn’t it funny how we sometimes do things like that? Keep the learning moving along today by checking out one or more of the following fun activities with a friend or family member:

  • Do you know there are a lot of fun games you can play with your friends that involve the sidewalk? From hopscotch to jump rope, there are all sorts of fun outside games you can play with your friends on the sidewalks in your neighborhood. For instructions — and maybe the chance to learn a new game or two — check out these links:
  • Get up close and personal with the sidewalks in your neighborhood. You may have walked on them for years and years without ever really noticing them, but today you’re going to get to know them. Get down on your hands and knees and examine the sidewalks in your area closely. Do you see the contraction joints? How long are they? Bring along a ruler or measuring stick to do some basic measurements. How far apart are the contraction joints? Do you notice any other cracks in the sidewalk? Have the contraction joints cracked in places? Bring along some water to test for cracks. Pour water along the contraction joints and watch for areas where it pools (solid) and areas where it disappears (cracks). Have fun putting what you learned in today’s Wonder of the Day to use in a scientific, practical way!
  • Up for a challenge? Are you a poet and don’t yet know it? We hope so! Your assignment…should you choose to accept it…is to write a poem FROM a sidewalk ON a sidewalk! Huh? Don’t worry. We’ll explain. This will be a piece of cake. First, pretend that you are a sidewalk. What is life like for a sidewalk? Use your imagination. What kinds of things do you see, hear and feel? Do you get tired of people walking on you? Or do you take delight in children riding their bikes or playing hopscotch on you? Write a short poem that shows what you think a sidewalk might say, think or feel. It doesn’t have to be very long. Since you’re going to write it on a sidewalk, you don’t want it to take up too much space. Here’s a quick example that we came up with:

Walk on me all day long.

I don’t mind. It’s fine.

Just try to keep your shoes clean.

I hate when dirt gets stuck in my lines!

We didn’t spend much time on our poem, so we’re sure you can do better if you use your imagination! When you’re finished, grab some sidewalk chalk and write your poem on a sidewalk where others will see it. If you would, please take a picture of your sidewalk poem and email it to us, post it on Facebook or tweet about it on Twitter. We can’t wait to read what you come up with! After sharing your creation, check out some of Shel Silverstein’s fun poetry and drawings from his book, Where the Sidewalk Ends.

Still Wondering

Check out Science NetLinks’ Water 3: Melting and Freezing lesson to explore what happens to the amount of different substances as they change from a solid to a liquid or a liquid to solid.


Test Your Knowledge

Wonder What’s Next?

We hope you join us in Wonderopolis tomorrow, even though it’s a bit cloudy!

Upload a Photo or Paste the URL of a YouTube or SchoolTube Video.