Why is this the answer?

The answer is teeth because this riddle is describing your beautiful, strong teeth! This is a wonderful riddle that uses word pictures to describe something you use every single day. Think about the first part of the riddle: "Soldiers line up spaced with pride. Two long rows lined side by side." If you look in the mirror, what do you see? Your teeth stand up straight and tall, lined up in neat, clean rows, just like soldiers standing in formation for a big parade. You have one long row of teeth on the top of your mouth, and a second, matching row on the bottom. They are spaced out perfectly next to each other, and that is why the riddle calls them "two long rows lined side by side." They are very important parts of your body, and they do a big, important job for you. Now let us think about the second part, which says, "One sole unit can decide, if the rows will unite or divide." This is the cleverest clue in the whole puzzle. When your mouth is relaxed or open, like when you are breathing or talking, your top and bottom rows of teeth are divided, or separated and far apart. But when you chew your food or bite into a crisp apple, the two rows of teeth must unite, or come together, to do the work of grinding and cutting up the food. The "one sole unit" that makes the decision to unite or divide the rows of teeth is your entire jaw and mouth system, which is controlled by your brain. You are the one who chooses to open or close your mouth. Your whole mouth system is like one commander that makes the decision to bring those two strong rows of tooth-soldiers together for a big job, or keep them apart so you can speak or yawn. When you eat, your teeth must unite to get the food ready for your tummy. When you make sounds like "sss" or "th," your teeth divide but get very close to help make the sound. This riddle paints a fun picture of the hard work your teeth do every day. They stand in their proud rows waiting for the command from you to get to work.