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!
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.